Supraventricular tachyarrhythmia discrimination

ABSTRACT

An implantable cardioverter defibrillator (ICD) performs a method that includes determining whether first criteria for detecting a ventricular tachyarrhythmia are met by a cardiac electrical signal. The ICD determines features from cardiac signal segment of a group of cardiac signal segments and determines whether a first portion of the features satisfy monomorphic waveform criteria and determines whether a second portion of the features satisfy supraventricular beat criteria. The ICD determines whether second criteria for detecting the ventricular tachyarrhythmia are met and withholds detecting of the ventricular tachyarrhythmia in response to the monomorphic waveform criteria and the supraventricular beat criteria being met.

TECHNICAL FIELD

The disclosure relates generally to an implantable medical device systemand method for discriminating supraventricular tachyarrhythmia fromventricular tachyarrhythmia.

BACKGROUND

Medical devices, such as cardiac pacemakers and implantable cardioverterdefibrillators (ICDs), provide therapeutic electrical stimulation to aheart of a patient via electrodes carried by one or more medicalelectrical leads and/or electrodes on a housing of the medical device.The electrical stimulation may include signals such as pacing pulses orcardioversion or defibrillation shocks. In some cases, a medical devicemay sense cardiac electrical signals attendant to the intrinsic orpacing-evoked depolarizations of the heart and control delivery ofstimulation signals to the heart based on sensed cardiac electricalsignals.

Upon detection of an abnormal rhythm, such as bradycardia, tachycardiaor fibrillation, an appropriate electrical stimulation signal or signalsmay be delivered to restore or maintain a more normal rhythm of theheart. For example, an ICD may deliver pacing pulses to the heart of thepatient upon detecting bradycardia or tachycardia or delivercardioversion or defibrillation shocks to the heart upon detectingtachycardia or fibrillation. The ICD may sense the cardiac electricalsignals in a heart chamber and deliver electrical stimulation therapiesto the heart chamber using electrodes carried by transvenous medicalelectrical leads. Cardiac signals sensed within the heart generally havea high signal strength and quality for reliably sensing cardiacelectrical events, such as R-waves. In other examples, a non-transvenouslead may be coupled to the ICD, in which case cardiac signal sensingpresents new challenges in accurately sensing cardiac electrical eventsand properly detecting and discriminating between different types ofcardiac arrhythmias.

Proper detection and discrimination of different tachyarrhythmias isimportant in automatically selecting and delivering an effectiveelectrical stimulation therapy by an implantable medical device systemand avoiding unnecessary therapies. For example, a supraventriculartachyarrhythmia originates in the upper, atrial heart chambers and isconducted to the lower, ventricular heart chambers. A supraventriculartachyarrhythmia (SVT) is generally not successfully terminated bydelivering electrical stimulation therapy to the ventricles because theheart rhythm is arising from the upper heart chambers. A ventriculartachyarrhythmia that originates in the lower, ventricular heartchambers, on the other hand, generally can be successfully treated bydelivering electrical stimulation therapies to the ventricles toterminate the abnormal ventricular rhythm. Accordingly, discriminationof supraventricular tachyarrhythmia that originates in the upper heartchambers from ventricular tachyarrhythmia that originates in the lowerheart chambers allows for appropriate therapy selection and deliverywhile avoiding unnecessary or potentially ineffective electricalstimulation therapy from being delivered to the patient's heart.

SUMMARY

In general, the disclosure is directed to techniques for discriminatingSVT from ventricular tachyarrhythmias, e.g., ventricular tachycardia(VT) and ventricular fibrillation (VF), and withholding VT and VFdetection and therapies when SVT is detected. In some examples, an ICDsystem operating according to the techniques disclosed herein maydetermine features of cardiac signal segments corresponding to sensedR-waves that occur at a tachyarrhythmia rate and have a morphologyindicative of ventricular tachycardia. A first group of the determinedcardiac signal segment features may be compared to monomorphic rhythmcriteria and a second group of the determined features may be comparedto SVT beat criteria. If the monomorphic rhythm criteria and the SVTbeat criteria are both satisfied, the rhythm may be identified as asupraventricular rhythm. Ventricular tachyarrhythmia detection andtherapy are delayed or withheld in response to detecting thesupraventricular rhythm.

In one example, the disclosure provides an ICD including a therapydelivery circuit, a sensing circuit and a control circuit coupled to thesensing circuit and the therapy delivery circuit. The therapy deliverycircuit is configured to generate an electrical stimulation therapy fordelivery to a patient's heart. The sensing circuit is configured toreceive a cardiac electrical signal via a sensing electrode vector. Thecontrol circuit is configured to determine whether first criteria fordetecting a ventricular tachyarrhythmia are met by the cardiacelectrical signal and determine features of each one of a group ofcardiac signal segments of the cardiac electrical signal. In response tothe first criteria being met, the control circuit determines whether afirst portion of the features determined from each one of the cardiacsignal segments satisfy monomorphic waveform criteria and determinewhether a second portion of the features determined from each one of thecardiac signal segments satisfy supraventricular beat criteria. Thecontrol circuit determines whether second criteria for detecting theventricular tachyarrhythmia are met and withholds detecting of theventricular tachyarrhythmia in response to both the monomorphic waveformcriteria being satisfied and the supraventricular beat criteria beingsatisfied. The control circuit detects the ventricular tachyarrhythmiaand controls the therapy delivery circuit to deliver the electricalstimulation therapy in response to the first criteria and the secondcriteria being met and at least one of the monomorphic waveform criterianot being satisfied and/or the supraventricular beat criteria not beingsatisfied.

In another example, the disclosure provides a method including receivingby a sensing circuit a cardiac electrical signal via a sensing electrodevector, determining by a control circuit whether first criteria fordetecting a ventricular tachyarrhythmia are met by the cardiacelectrical signal, determining features of each one of a group ofcardiac signal segments of the cardiac electrical signal and, inresponse to the first criteria being met, determining whether a firstportion of the features determined from each one of the cardiac signalsegments satisfy monomorphic waveform criteria and whether a secondportion of the features determined from each one of the cardiac signalsegments satisfy supraventricular beat criteria. The method furtherincludes determining whether second criteria for detecting theventricular tachyarrhythmia are met, determining whether both the firstportion of the plurality of features satisfy the monomorphic waveformcriteria and the second portion of the plurality of features satisfy thesupraventricular beat criteria, and withholding detecting of theventricular tachyarrhythmia in response both the first portion of theplurality of features satisfying the monomorphic waveform criteria andthe second portion of the plurality of features satisfying thesupraventricular beat criteria. The method includes detecting theventricular tachyarrhythmia and controlling the therapy delivery circuitto deliver the electrical stimulation therapy in response to the firstcriteria and the second criteria being met and at least one of the firstportion of the plurality of features not satisfying the monomorphicwaveform criteria and/or the second portion of the plurality of featuresnot satisfying the supraventricular beat criteria.

In another example, the disclosure provides a non-transitory,computer-readable storage medium storing a set of instructions which,when executed by a control circuit of an ICD, cause the ICD to receiveby a sensing circuit a cardiac electrical signal via a sensing electrodevector; determine whether first criteria for detecting a ventriculartachyarrhythmia are met by the cardiac electrical signal;

determine features of each one of a group of cardiac signal segments ofthe cardiac electrical signal; in response to the first criteria beingmet, determine whether a first portion of the features determined fromeach one of the cardiac signal segments satisfy monomorphic waveformcriteria; determine whether a second portion of the features determinedfrom each one of the cardiac signal segments satisfy supraventricularbeat criteria; determine whether second criteria for detecting theventricular tachyarrhythmia are met; withhold detecting of theventricular tachyarrhythmia in response to both the monomorphic waveformcriteria being satisfied and the supraventricular beat criteria beingsatisfied; and detect the ventricular tachyarrhythmia and deliver anelectrical stimulation therapy by a therapy delivery circuit in responseto the first criteria and the second criteria being met and at least oneof the monomorphic waveform criteria not being satisfied or thesupraventricular beat criteria not being satisfied.

This summary is intended to provide an overview of the subject matterdescribed in this disclosure. It is not intended to provide an exclusiveor exhaustive explanation of the apparatus and methods described indetail within the accompanying drawings and description below. Furtherdetails of one or more examples are set forth in the accompanyingdrawings and the description below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are conceptual diagrams of an extra-cardiovascular ICDsystem according to one example.

FIGS. 2A-2C are conceptual diagrams of a patient implanted with theextra-cardiovascular ICD system of FIG. 1A in a different implantconfiguration.

FIG. 3 is a schematic diagram of the ICD of FIGS. 1A-2C according to oneexample.

FIG. 4 is diagram of circuitry included in the sensing circuit of FIG. 3according to one example.

FIG. 5 is a flow chart of a method performed by an ICD fordiscriminating SVT from a ventricular tachyarrhythmia according to oneexample.

FIG. 6 is a flow chart of a method that may be performed by an ICD forestablishing an SVT morphology template.

FIG. 7 is a diagram of one example of a cardiac signal segment fromwhich cardiac signal segment features are determined when SVCdiscrimination is enabled.

FIG. 8 is a diagram of an example cardiac signal segment having amonophasic polarity pattern.

FIG. 9 is a schematic diagram of a method for determining features froma cardiac signal segment for use in determining if monomorphic waveformcriteria are satisfied.

FIG. 10 is a flow chart of a method for discriminating SVT from VT/VFand for adjusting VT/VF morphology criteria according to anotherexample.

FIG. 11 is a flow chart of a for determining if monomorphic waveformcriteria are satisfied according to one example.

FIG. 12 is a flow chart for determining if SVT beat criteria aresatisfied.

FIG. 13 is a flow chart of a method for detecting ventriculartachyarrhythmias according to one example using the SVT discriminationtechniques disclosed herein.

FIG. 14 is a flow chart of a method for detecting ventriculartachyarrhythmias according to another example.

DETAILED DESCRIPTION

In general, this disclosure describes techniques for discriminating SVTfrom VT and VF by a cardiac medical device or system and withholdingdetection of a ventricular tachyarrhythmia in response to detecting SVT.Criteria for detecting ventricular tachyarrhythmia, such as heartrate-based criteria, may become satisfied in the presence of SVT. Assuch, heart rate alone may be insufficient for reliably discriminatingbetween SVT and VT/VF. Techniques for detecting SVT as described hereinallow a tachyarrhythmia detection to be withheld when evidence of SVT isidentified.

In some examples, the cardiac medical device system may be anextra-cardiovascular ICD system. As used herein, the term“extra-cardiovascular” refers to a position outside the blood vessels,heart, and pericardium surrounding the heart of a patient. Implantableelectrodes carried by extra-cardiovascular leads may be positionedextra-thoracically (outside the ribcage and sternum) orintra-thoracically (beneath the ribcage or sternum) but generally not inintimate contact with myocardial tissue. The techniques disclosed hereinfor detecting SVT and withholding a VT/VF detection may be applied to acardiac electrical signal acquired using extra-cardiovascularelectrodes.

These techniques are presented herein in conjunction with an ICD andimplantable medical lead carrying extra-cardiovascular electrodes, butaspects disclosed herein may be utilized in conjunction with othercardiac medical devices or systems. For example, the techniques fordetecting SVT as described in conjunction with the accompanying drawingsmay be implemented in any implantable or external medical device enabledfor sensing cardiac electrical signals, including implantablepacemakers, ICDs or cardiac monitors coupled to transvenous, pericardialor epicardial leads carrying sensing and therapy delivery electrodes;leadless pacemakers, ICDs or cardiac monitors having housing-basedsensing electrodes; and external or wearable pacemakers, defibrillators,or cardiac monitors coupled to external, surface or skin electrodes.

FIGS. 1A and 1B are conceptual diagrams of an extra-cardiovascular ICDsystem 10 according to one example. FIG. 1A is a front view of ICDsystem 10 implanted within patient 12. FIG. 1B is a side view of ICDsystem 10 implanted within patient 12. ICD system 10 includes an ICD 14connected to an extra-cardiovascular electrical stimulation and sensinglead 16. FIGS. 1A and 1B are described in the context of an ICD system10 capable of providing defibrillation and/or cardioversion shocks andpacing pulses.

ICD 14 includes a housing 15 that forms a hermetic seal that protectsinternal components of ICD 14. The housing 15 of ICD 14 may be formed ofa conductive material, such as titanium or titanium alloy. The housing15 may function as an electrode (sometimes referred to as a “can”electrode). Housing 15 may be used as an active can electrode for use indelivering cardioversion/defibrillation (CV/DF) shocks or other highvoltage pulses delivered using a high voltage therapy circuit. In otherexamples, housing 15 may be available for use in delivering unipolar,low voltage cardiac pacing pulses and/or for sensing cardiac electricalsignals in combination with electrodes carried by lead 16. In otherinstances, the housing 15 of ICD 14 may include a plurality ofelectrodes on an outer portion of the housing. The outer portion(s) ofthe housing 15 functioning as an electrode(s) may be coated with amaterial, such as titanium nitride, e.g., for reducing post-stimulationpolarization artifact.

ICD 14 includes a connector assembly 17 (also referred to as a connectorblock or header) that includes electrical feedthroughs crossing housing15 to provide electrical connections between conductors extending withinthe lead body 18 of lead 16 and electronic components included withinthe housing 15 of ICD 14. As will be described in further detail herein,housing 15 may house one or more processors, memories, transceivers,electrical cardiac signal sensing circuitry, therapy delivery circuitry,power sources and other components for sensing cardiac electricalsignals, detecting a heart rhythm, and controlling and deliveringelectrical stimulation pulses to treat an abnormal heart rhythm.

Elongated lead body 18 has a proximal end 27 that includes a leadconnector (not shown) configured to be connected to ICD connectorassembly 17 and a distal portion 25 that includes one or moreelectrodes. In the example illustrated in FIGS. 1A and 1B, the distalportion 25 of lead body 18 includes defibrillation electrodes 24 and 26and pace/sense electrodes 28 and 30. In some cases, defibrillationelectrodes 24 and 26 may together form a defibrillation electrode inthat they may be configured to be activated concurrently. Alternatively,defibrillation electrodes 24 and 26 may form separate defibrillationelectrodes in which case each of the electrodes 24 and 26 may beactivated independently.

Electrodes 24 and 26 (and in some examples housing 15) are referred toherein as defibrillation electrodes because they are utilized,individually or collectively, for delivering high voltage stimulationtherapy (e.g., cardioversion or defibrillation shocks). Electrodes 24and 26 may be elongated coil electrodes and generally have a relativelyhigh surface area for delivering high voltage electrical stimulationpulses compared to pacing and sensing electrodes 28 and 30. However,electrodes 24 and 26 and housing 15 may also be utilized to providepacing functionality, sensing functionality or both pacing and sensingfunctionality in addition to or instead of high voltage stimulationtherapy. In this sense, the use of the term “defibrillation electrode”herein should not be considered as limiting the electrodes 24 and 26 foruse in only high voltage cardioversion/defibrillation shock therapyapplications. For example, electrodes 24 and 26 may be used in a sensingvector used to sense cardiac electrical signals and detect anddiscriminate SVT, VT and VF.

Electrodes 28 and 30 are relatively smaller surface area electrodeswhich are available for use in sensing electrode vectors for sensingcardiac electrical signals and may be used for delivering relatively lowvoltage pacing pulses in some configurations. Electrodes 28 and 30 arereferred to as pace/sense electrodes because they are generallyconfigured for use in low voltage applications, e.g., used as either acathode or anode for delivery of pacing pulses and/or sensing of cardiacelectrical signals, as opposed to delivering high voltage cardioversiondefibrillation shocks. In some instances, electrodes 28 and 30 mayprovide only pacing functionality, only sensing functionality or both.

ICD 14 may obtain cardiac electrical signals corresponding to electricalactivity of heart 8 via a combination of sensing vectors that includecombinations of electrodes 24, 26, 28 and/or 30. In some examples,housing 15 of ICD 14 is used in combination with one or more ofelectrodes 24, 26, 28 and/or 30 in a sensing electrode vector. Varioussensing electrode vectors utilizing combinations of electrodes 24, 26,28, and 30 and housing 15 are described below for acquiring first andsecond cardiac electrical signals using respective first and secondsensing electrode vectors that may be selected by sensing circuitryincluded in ICD 14.

In the example illustrated in FIGS. 1A and 1B, electrode 28 is locatedproximal to defibrillation electrode 24, and electrode 30 is locatedbetween defibrillation electrodes 24 and 26. One, two or more pace/senseelectrodes may be carried by lead body 18. For instance, a thirdpace/sense electrode may be located distal to defibrillation electrode26 in some examples. Electrodes 28 and 30 are illustrated as ringelectrodes; however, electrodes 28 and 30 may comprise any of a numberof different types of electrodes, including ring electrodes, short coilelectrodes, hemispherical electrodes, directional electrodes, segmentedelectrodes, or the like. Electrodes 28 and 30 may be positioned at anylocation along lead body 18 and are not limited to the positions shown.In other examples, lead 16 may include none, one or more pace/senseelectrodes and/or one or more defibrillation electrodes.

In the example shown, lead 16 extends subcutaneously or submuscularlyover the ribcage 32 medially from the connector assembly 27 of ICD 14toward a center of the torso of patient 12, e.g., toward xiphoid process20 of patient 12. At a location near xiphoid process 20, lead 16 bendsor turns and extends superior subcutaneously or submuscularly over theribcage and/or sternum, substantially parallel to sternum 22. Althoughillustrated in FIG. 1A as being offset laterally from and extendingsubstantially parallel to sternum 22, the distal portion 25 of lead 16may be implanted at other locations, such as over sternum 22, offset tothe right or left of sternum 22, angled laterally from sternum 22 towardthe left or the right, or the like. Alternatively, lead 16 may be placedalong other subcutaneous or submuscular paths. The path ofextra-cardiovascular lead 16 may depend on the location of ICD 14, thearrangement and position of electrodes carried by the lead body 18,and/or other factors.

Electrical conductors (not illustrated) extend through one or morelumens of the elongated lead body 18 of lead 16 from the lead connectorat the proximal lead end 27 to electrodes 24, 26, 28, and 30 locatedalong the distal portion 25 of the lead body 18. The elongatedelectrical conductors contained within the lead body 18 are eachelectrically coupled with respective defibrillation electrodes 24 and 26and pace/sense electrodes 28 and 30, which may be separate respectiveinsulated conductors within the lead body 18. The respective conductorselectrically couple the electrodes 24, 26, 28, and 30 to circuitry, suchas a therapy delivery circuit and/or a sensing circuit, of ICD 14 viaconnections in the connector assembly 17, including associatedelectrical feedthroughs crossing housing 15. The electrical conductorstransmit therapy from a therapy delivery circuit within ICD 14 to one ormore of defibrillation electrodes 24 and 26 and/or pace/sense electrodes28 and 30 and transmit sensed electrical signals from one or more ofdefibrillation electrodes 24 and 26 and/or pace/sense electrodes 28 and30 to the sensing circuit within ICD 14.

The lead body 18 of lead 16 may be formed from a non-conductivematerial, including silicone, polyurethane, fluoropolymers, mixturesthereof, and other appropriate materials, and shaped to form one or morelumens within which the one or more conductors extend. Lead body 18 maybe tubular or cylindrical in shape. In other examples, the distalportion 25 (or all of) the elongated lead body 18 may have a flat,ribbon or paddle shape. Lead body 18 may be formed having a preformeddistal portion 25 that is generally straight, curving, bending,serpentine, undulating or zig-zagging.

In the example shown, lead body 18 includes a curving distal portion 25having two “C” shaped curves, which together may resemble the Greekletter epsilon, “ε.” Defibrillation electrodes 24 and 26 are eachcarried by one of the two respective C-shaped portions of the lead bodydistal portion 25. The two C-shaped curves are seen to extend or curvein the same direction away from a central axis of lead body 18, alongwhich pace/sense electrodes 28 and 30 are positioned. Pace/senseelectrodes 28 and 30 may, in some instances, be approximately alignedwith the central axis of the straight, proximal portion of lead body 18such that mid-points of defibrillation electrodes 24 and 26 arelaterally offset from pace/sense electrodes 28 and 30.

Other examples of extra-cardiovascular leads including one or moredefibrillation electrodes and one or more pacing and sensing electrodescarried by curving, serpentine, undulating or zig-zagging distal portionof the lead body 18 that may be implemented with the techniquesdescribed herein are generally disclosed in pending U.S. Pat.Publication No. 2016/0158567 (Marshall, et al.), incorporated herein byreference in its entirety. The techniques disclosed herein are notlimited to any particular lead body design, however. In other examples,lead body 18 is a flexible elongated lead body without any pre-formedshape, bends or curves. Various example configurations ofextra-cardiovascular leads and electrodes and dimensions that may beimplemented in conjunction with the SVT discrimination techniquesdisclosed herein are described in pending U.S. Publication No.2015/0306375 (Marshall, et al.) and pending U.S. Publication No.2015/0306410 (Marshall, et al.), both of which are incorporated hereinby reference in their entirety.

ICD 14 analyzes the cardiac electrical signals received from one or moresensing electrode vectors to monitor for abnormal rhythms, such asbradycardia, SVT, VT or VF. ICD 14 may analyze the heart rate andmorphology of the cardiac electrical signals to monitor fortachyarrhythmia in accordance with any of a number of tachyarrhythmiadetection techniques. One example technique for detectingtachyarrhythmia is described in U.S. Pat. No. 7,761,150 (Ghanem, etal.), incorporated herein by reference in its entirety. Exampletechniques for detecting VT and VF are described below in conjunctionwith the accompanying figures. The techniques for discriminating SVTfrom VT or VF for withholding a VT or VF detection as disclosed hereinmay be incorporated in a variety of VT/VF detection algorithms. Examplesof devices and tachyarrhythmia detection algorithms that may be adaptedto utilize techniques for SVT discrimination described herein aregenerally disclosed in U.S. Pat. No. 5,354,316 (Keimel); U.S. Pat. No.5,545,186 (Olson, et al.); U.S. Pat. No. 6,393,316 (Gillberg et al.);U.S. Pat. No. 7,031,771 (Brown, et al.); U.S. Pat. No. 8,160,684(Ghanem, et al.), and U.S. Pat. No. 8,437,842 (Zhang, et al.), all ofwhich patents are incorporated herein by reference in their entirety.

ICD 14 generates and delivers electrical stimulation therapy in responseto detecting a tachyarrhythmia (e.g., VT or VF) using a therapy deliveryelectrode vector which may be selected from any of the availableelectrodes 24, 26, 28 30 and/or housing 15. ICD 14 may deliver ATP inresponse to VT detection, and in some cases may deliver ATP prior to aCV/DF shock or during high voltage capacitor charging in an attempt toavert the need for delivering a CV/DF shock. If ATP does notsuccessfully terminate VT or when VF is detected, ICD 14 may deliver oneor more CV/DF shocks via one or both of defibrillation electrodes 24 and26 and/or housing 15. ICD 14 may deliver the CV/DF shocks usingelectrodes 24 and 26 individually or together as a cathode (or anode)and with the housing 15 as an anode (or cathode). ICD 14 may generateand deliver other types of electrical stimulation pulses such aspost-shock pacing pulses or bradycardia pacing pulses using a pacingelectrode vector that includes one or more of the electrodes 24, 26, 28,and 30 and the housing 15 of ICD 14.

FIGS. 1A and 1B are illustrative in nature and should not be consideredlimiting of the practice of the techniques disclosed herein. ICD 14 isshown implanted subcutaneously on the left side of patient 12 along theribcage 32. ICD 14 may, in some instances, be implanted between the leftposterior axillary line and the left anterior axillary line of patient12. ICD 14 may, however, be implanted at other subcutaneous orsubmuscular locations in patient 12. For example, ICD 14 may beimplanted in a subcutaneous pocket in the pectoral region. In this case,lead 16 may extend subcutaneously or submuscularly from ICD 14 towardthe manubrium of sternum 22 and bend or turn and extend inferiorly fromthe manubrium to the desired location subcutaneously or submuscularly.In yet another example, ICD 14 may be placed abdominally. Lead 16 may beimplanted in other extra-cardiovascular locations as well. For instance,as described with respect to FIGS. 2A-2C, the distal portion 25 of lead16 may be implanted underneath the sternum/ribcage in the substernalspace.

An external device 40 is shown in telemetric communication with ICD 14by a communication link 42. External device 40 may include a processor,display, user interface, telemetry unit and other components forcommunicating with ICD 14 for transmitting and receiving data viacommunication link 42. Communication link 42 may be established betweenICD 14 and external device 40 using a radio frequency (RF) link such asBLUETOOTH®, Wi-Fi, or Medical Implant Communication Service (MICS) orother RF or communication frequency bandwidth.

External device 40 may be embodied as a programmer used in a hospital,clinic or physician's office to retrieve data from ICD 14 and to programoperating parameters and algorithms in ICD 14 for controlling ICDfunctions. External device 40 may be used to program cardiac eventsensing parameters (e.g., R-wave sensing parameters), cardiac rhythmdetection parameters (e.g., VT and VF detection parameters and SVTdiscrimination parameters) and therapy control parameters used by ICD14. Data stored or acquired by ICD 14, including physiological signalsor associated data derived therefrom, results of device diagnostics, andhistories of detected rhythm episodes and delivered therapies, may beretrieved from ICD 14 by external device 40 following an interrogationcommand. External device 40 may alternatively be embodied as a homemonitor or hand held device.

FIGS. 2A-2C are conceptual diagrams of patient 12 implanted withextra-cardiovascular ICD system 10 in a different implant configurationthan the arrangement shown in FIGS. 1A-1B. FIG. 2A is a front view ofpatient 12 implanted with ICD system 10. FIG. 2B is a side view ofpatient 12 implanted with ICD system 10. FIG. 2C is a transverse view ofpatient 12 implanted with ICD system 10. In this arrangement,extra-cardiovascular lead 16 of system 10 is implanted at leastpartially underneath sternum 22 of patient 12. Lead 16 extendssubcutaneously or submuscularly from ICD 14 toward xiphoid process 20and at a location near xiphoid process 20 bends or turns and extendssuperiorly within anterior mediastinum 36 in a substernal position.

Anterior mediastinum 36 may be viewed as being bounded laterally bypleurae 39, posteriorly by pericardium 38, and anteriorly by sternum 22(see FIG. 2C). The distal portion 25 of lead 16 may extend along theposterior side of sternum 22 substantially within the loose connectivetissue and/or substernal musculature of anterior mediastinum 36. A leadimplanted such that the distal portion 25 is substantially withinanterior mediastinum 36, may be referred to as a “substernal lead.”

In the example illustrated in FIGS. 2A-2C, lead 16 is locatedsubstantially centered under sternum 22. In other instances, however,lead 16 may be implanted such that it is offset laterally from thecenter of sternum 22. In some instances, lead 16 may extend laterallysuch that distal portion 25 of lead 16 is underneath/below the ribcage32 in addition to or instead of sternum 22. In other examples, thedistal portion 25 of lead 16 may be implanted in otherextra-cardiovascular, intra-thoracic locations, including the pleuralcavity or around the perimeter of and adjacent to but typically notwithin the pericardium 38 of heart 8. Other implant locations and leadand electrode arrangements that may be used in conjunction with the SVTdiscrimination techniques described herein are generally disclosed inthe above-incorporated references.

FIG. 3 is a schematic diagram of ICD 14 according to one example. Theelectronic circuitry enclosed within housing 15 (shown schematically asan electrode in FIG. 3) includes software, firmware and hardware thatcooperatively monitor cardiac electrical signals, determine when anelectrical stimulation therapy is necessary, and deliver therapies asneeded according to programmed therapy delivery algorithms and controlparameters. The software, firmware and hardware are configured to detecttachyarrhythmias and deliver anti-tachyarrhythmia therapy, e.g., detectventricular tachyarrhythmias and in some cases discriminate VT from VFfor determining when ATP or CV/DF shocks are required. ICD 14 is coupledto an extra-cardiovascular lead, such as lead 16 carryingextra-cardiovascular electrodes 24, 26, 28, and 30, for deliveringelectrical stimulation pulses to the patient's heart and for sensingcardiac electrical signals.

ICD 14 includes a control circuit 80, memory 82, therapy deliverycircuit 84, sensing circuit 86, and telemetry circuit 88. A power source98 provides power to the circuitry of ICD 14, including each of thecomponents 80, 82, 84, 86, and 88 as needed. Power source 98 may includeone or more energy storage devices, such as one or more rechargeable ornon-rechargeable batteries. The connections between power source 98 andeach of the other components 80, 82, 84, 86 and 88 are to be understoodfrom the general block diagram of FIG. 3, but are not shown for the sakeof clarity. For example, power source 98 may be coupled to one or morecharging circuits included in therapy delivery circuit 84 for chargingholding capacitors included in therapy delivery circuit 84 that aredischarged at appropriate times under the control of control circuit 80for producing electrical pulses according to a therapy protocol, such asfor bradycardia pacing, post-shock pacing, ATP and/or CV/DF shockpulses. Power source 98 is also coupled to components of sensing circuit86, such as sense amplifiers, analog-to-digital converters, switchingcircuitry, etc. as needed.

The functional blocks shown in FIG. 3 represent functionality includedin ICD 14 and may include any discrete and/or integrated electroniccircuit components that implement analog and/or digital circuits capableof producing the functions attributed to ICD 14 herein. The variouscomponents may include an application specific integrated circuit(ASIC), an electronic circuit, a processor (shared, dedicated, or group)and memory that execute one or more software or firmware programs, acombinational logic circuit, state machine, or other suitable componentsor combinations of components that provide the described functionality.The particular form of software, hardware and/or firmware employed toimplement the functionality disclosed herein will be determinedprimarily by the particular system architecture employed in the ICD andby the particular detection and therapy delivery methodologies employedby the ICD. Providing software, hardware, and/or firmware to accomplishthe described functionality in the context of any modern ICD system,given the disclosure herein, is within the abilities of one of skill inthe art.

Memory 82 may include any volatile, non-volatile, magnetic, orelectrical non-transitory computer readable storage media, such asrandom access memory (RAM), read-only memory (ROM), non-volatile RAM(NVRAM), electrically-erasable programmable ROM (EEPROM), flash memory,or any other memory device. Furthermore, memory 82 may includenon-transitory computer readable media storing instructions that, whenexecuted by one or more processing circuits, cause control circuit 80and/or other ICD components to perform various functions attributed toICD 14 or those ICD components. The non-transitory computer-readablemedia storing the instructions may include any of the media listedabove.

The functions attributed to ICD 14 herein may be embodied as one or moreintegrated circuits. Depiction of different features as circuits isintended to highlight different functional aspects and does notnecessarily imply that such circuits must be realized by separatehardware or software components. Rather, functionality associated withone or more circuits may be performed by separate hardware, firmware orsoftware components, or integrated within common hardware, firmware orsoftware components. For example, cardiac event sensing andtachyarrhythmia detection operations may be performed cooperatively bysensing circuit 86 and control circuit 80 and may include operationsimplemented in a processor or other signal processing circuitry includedin control circuit 80 executing instructions stored in memory 82 andcontrol signals such as blanking and timing intervals and sensingthreshold amplitude signals sent from control circuit 80 to sensingcircuit 86.

Control circuit 80 communicates, e.g., via a data bus, with therapydelivery circuit 84 and sensing circuit 86 for sensing cardiacelectrical activity, detecting cardiac rhythms, and controlling deliveryof cardiac electrical stimulation therapies in response to sensedcardiac signals. Therapy delivery circuit 84 and sensing circuit 86 areelectrically coupled to electrodes 24, 26, 28, 30 carried by lead 16 andthe housing 15, which may function as a common or ground electrode or asan active can electrode for delivering CV/DF shock pulses or cardiacpacing pulses.

Sensing circuit 86 may be selectively coupled to electrodes 28, 30and/or housing 15 in order to monitor electrical activity of thepatient's heart. Sensing circuit 86 may additionally be selectivelycoupled to defibrillation electrodes 24 and/or 26 for use in a sensingelectrode vector together or in combination with one or more ofelectrodes 28, 30 and/or housing 15. Sensing circuit 86 may be enabledto selectively receive cardiac electrical signals from at least twosensing electrode vectors from the available electrodes 24, 26, 28, 30,and housing 15. At least two cardiac electrical signals from twodifferent sensing electrode vectors may be received simultaneously bysensing circuit 86. Sensing circuit 86 may monitor one or both or thecardiac electrical signals at a time for sensing cardiac electricalevents, e.g., P-waves attendant to the depolarization of the atrialmyocardium and/or R-waves attendant to the depolarization of theventricular myocardium, and providing digitized cardiac signal waveformsfor analysis by control circuit 80. For example, sensing circuit 86 mayinclude switching circuitry (not shown) for selecting which ofelectrodes 24, 26, 28, 30, and housing 15 are coupled to a first sensingchannel 83 and which are coupled to a second sensing channel 85 ofsensing circuit 86. Switching circuitry may include a switch array,switch matrix, multiplexer, or any other type of switching devicesuitable to selectively couple components of sensing circuit 86 toselected electrodes.

Each sensing channel 83 and 85 may be configured to amplify, filter anddigitize the cardiac electrical signal received from selected electrodescoupled to the respective sensing channel to improve the signal qualityfor detecting cardiac electrical events, such as R-waves or performingother signal analysis. The cardiac event detection circuitry withinsensing circuit 86 may include one or more sense amplifiers, filters,rectifiers, threshold detectors, comparators, analog-to-digitalconverters (ADCs), timers or other analog or digital components asdescribed further in conjunction with FIG. 4. A cardiac event sensingthreshold may be automatically adjusted by sensing circuit 86 under thecontrol of control circuit 80, based on timing intervals and sensingthreshold values determined by control circuit 80, stored in memory 82,and/or controlled by hardware, firmware and/or software of controlcircuit 80 and/or sensing circuit 86.

Upon detecting a cardiac electrical event based on a sensing thresholdcrossing, sensing circuit 86 may produce a sensed event signal, such asan R-wave sensed event signal, that is passed to control circuit 80. Insome examples, the sensed event signal may be used by control circuit 80to trigger storage of a segment of a cardiac electrical signal foranalysis for confirming the R-wave sensed event signals anddiscriminating SVT as described below.

The R-wave sensed event signals are also used by control circuit 80 fordetermining RR intervals (RRIs) for detecting tachyarrhythmia anddetermining a need for therapy. An RRI is the time interval between twoconsecutively sensed R-waves and may be determined between twoconsecutive R-wave sensed event signals received from sensing circuit86. For example, control circuit 80 may include a timing circuit 90 fordetermining RRIs between consecutive R-wave sensed event signalsreceived from sensing circuit 86 and for controlling various timersand/or counters used to control the timing of therapy delivery bytherapy delivery circuit 84. Timing circuit 90 may additionally set timewindows such as morphology template windows, morphology analysis windowsor perform other timing related functions of ICD 14 includingsynchronizing CV/DF shocks or other therapies delivered by therapydelivery circuit 84 with sensed cardiac events. Tachyarrhythmia detector92 is configured to analyze signals received from sensing circuit 86 fordetecting tachyarrhythmia episodes. Tachyarrhythmia detector 92 may beimplemented in control circuit 80 as software, hardware and/or firmwarethat processes and analyzes signals received from sensing circuit 86 fordetecting VT and/or VF. In some examples, tachyarrhythmia detector 92may include comparators and counters for counting RRIs determined bytiming circuit 92 that fall into various rate detection zones fordetermining a ventricular rate or performing other rate- orinterval-based assessments for detecting and discriminating VT and VF.For example, tachyarrhythmia detector 92 may compare the RRIs determinedby timing circuit 90 to one or more tachyarrhythmia detection intervalzones, such as a tachycardia detection interval zone and a fibrillationdetection interval zone. RRIs falling into a detection interval zone arecounted by a respective VT interval counter or VF interval counter andin some cases in a combined VT/VF interval counter included intachyarrhythmia detector 92.

When a VT or VF interval counter reaches a threshold count value, oftenreferred to as “number of intervals to detect” or “NID,” a ventriculartachyarrhythmia may be detected by control circuit 80. Tachyarrhythmiadetector 92 may be configured to perform other signal analysis fordetermining if other detection criteria are satisfied before detectingVT or VF when an NID is reached however. For example, cardiac signalanalysis may be performed to determine if R-wave morphology criteria,onset criteria, and noise and oversensing rejection criteria aresatisfied in order to determine if the VT/VF detection should be made orwithheld. As disclosed herein, tachyarrhythmia detector 92 may withholdthe VT or VF detection when an NID is reached if analysis of cardiacsignal waveform features indicates that the rhythm is an SVT rhythm.

To support additional cardiac signal analyses performed bytachyarrhythmia detector 92, sensing circuit 86 may pass a digitizedcardiac electrical signal to control circuit 80. A cardiac electricalsignal from the selected sensing channel, e.g., from first sensingchannel 83 and/or the second sensing channel 85, may be passed through afilter and amplifier, provided to a multiplexer and thereafter convertedto multi-bit digital signals by an analog-to-digital converter, allincluded in sensing circuit 86, for storage in memory 82.

Memory 82 may include read-only memory (ROM) in which stored programscontrolling the operation of the control circuit 80 reside. Memory 82may further include random access memory (RAM) or other memory devicesconfigured as a number of recirculating buffers capable of holding aseries of measured RRIs, counts or other data for analysis by thetachyarrhythmia detector 92. Memory 82 may be configured to store apredetermined number of cardiac electrical signal segments incirculating buffers under the control of control circuit 80. Forinstance, up to eight cardiac electrical signal segments eachcorresponding to an R-wave sensed event signal may be stored in memory82. Additionally or alternatively, features derived from each of up toeight cardiac signal segments that each correspond to an R-wave sensedevent signal may be buffered in memory 82 for use in SVT discriminationas described below.

Therapy delivery circuit 84 includes charging circuitry, one or morecharge storage devices such as one or more high voltage capacitorsand/or low voltage capacitors, and switching circuitry that controlswhen the capacitor(s) are discharged across a selected pacing electrodevector or CV/DF shock vector. Charging of capacitors to a programmedpulse amplitude and discharging of the capacitors for a programmed pulsewidth may be performed by therapy delivery circuit 84 according tocontrol signals received from control circuit 80. Timing circuit 90 ofcontrol circuit 80 may include various timers or counters that controlwhen ATP or other cardiac pacing pulses are delivered. For example,timing circuit 90 may include programmable digital counters set by amicroprocessor of the control circuit 80 for controlling the basicpacing time intervals associated with various pacing modes or ATPsequences delivered by ICD 14. The microprocessor of control circuit 80may also set the amplitude, pulse width, polarity or othercharacteristics of the cardiac pacing pulses, which may be based onprogrammed values stored in memory 82.

In response to detecting VT or VF, control circuit 80 may controltherapy delivery circuit 84 to deliver therapies such as ATP and/orCV/DF therapy. Therapy can be delivered by initiating charging of highvoltage capacitors via a charging circuit, both included in therapydelivery circuit 84. Charging is controlled by control circuit 80 whichmonitors the voltage on the high voltage capacitors, which is passed tocontrol circuit 80 via a charging control line. When the voltage reachesa predetermined value set by control circuit 80, a logic signal isgenerated on a capacitor full line and passed to therapy deliverycircuit 84, terminating charging. A CV/DF pulse is delivered to theheart under the control of the timing circuit 90 by an output circuit oftherapy delivery circuit 84 via a control bus. The output circuit mayinclude an output capacitor through which the charged high voltagecapacitor is discharged via switching circuitry, e.g., an H-bridge,which determines the electrodes used for delivering the cardioversion ordefibrillation pulse and the pulse wave shape. In some examples, thehigh voltage therapy circuit configured to deliver CV/DF shock pulsescan be controlled by control circuit 80 to deliver pacing pulses, e.g.,for delivering ATP or post shock pacing pulses. In other examples,therapy delivery circuit 84 may include a low voltage therapy circuitfor generating and delivering relatively lower voltage pacing pulses fora variety of pacing needs. Therapy delivery and control circuitrygenerally disclosed in any of the above-incorporated patents may beimplemented in ICD 14.

It is recognized that the methods disclosed herein may be implemented inan implantable medical device that is used for monitoring cardiacelectrical signals by sensing circuit 86 and control circuit 80 withouthaving therapy delivery capabilities or in an implantable medical devicethat monitors cardiac electrical signals and delivers cardiac pacingtherapies by therapy delivery circuit 84, without high voltage therapycapabilities, such as cardioversion/defibrillation shock capabilities orvice versa.

Control parameters utilized by control circuit 80 for detecting cardiacarrhythmias and controlling therapy delivery may be programmed intomemory 82 via telemetry circuit 88. Telemetry circuit 88 includes atransceiver and antenna for communicating with external device 40 (shownin FIG. 1A) using RF communication or other communication protocols asdescribed above. Under the control of control circuit 80, telemetrycircuit 88 may receive downlink telemetry from and send uplink telemetryto external device 40. In some cases, telemetry circuit 88 may be usedto transmit and receive communication signals to/from another medicaldevice implanted in patient 12.

FIG. 4 is a diagram of circuitry included in first sensing channel 83and second sensing channel 85 of sensing circuit 86 according to oneexample. First sensing channel 83 may be selectively coupled viaswitching circuitry 61 to a first sensing electrode vector includingelectrodes carried by extra-cardiovascular lead 16 as shown in FIGS.1A-20 for receiving a first cardiac electrical signal. First sensingchannel 83 may be coupled to a sensing electrode vector that is a shortbipole, having a relatively shorter inter-electrode distance or spacingthan the second electrode vector coupled to second sensing channel 85.For example, the first sensing electrode vector may include pace/senseelectrodes 28 and 30. In other examples, the first sensing electrodevector coupled to sensing channel 83 may include a defibrillationelectrode 24 and/or 26, e.g., a sensing electrode vector betweenpace/sense electrode 28 and defibrillation electrode 24 or betweenpace/sense electrode 30 and either of defibrillation electrodes 24 or26. In still other examples, the first sensing electrode vector may bebetween defibrillation electrodes 24 and 26.

In some patients, a bipole between electrodes carried by lead 16 mayresult in patient body posture dependent changes in the cardiacelectrical signal as the sensing vector of the bipole relative to thecardiac axis changes with changes in patient body posture or bodymotion. Accordingly, the sensing electrode vector coupled to the firstsensing channel 83 may include housing 15 and any of the electrodes 24,26, 28 and 30 carried by lead 16. A relatively longer bipole includinghousing 15 and a lead-based electrode may be less sensitive topositional changes. Cardiac electrical signals received viaextra-cardiovascular electrodes may be influenced by positional changesof the patient than electrodes carried by transvenous leads. Theamplitude, polarity, and wave shape of R-waves may change, for example,as patient posture changes. As a result, R-wave morphology analysisperformed to discriminate between SVT and VT/VF may lead to false VT/VFdetection when R-wave amplitude and/or morphology has changed due topositional changes of the patient. The techniques disclosed herein maybe used to detect and discriminate SVT to avoid false detection of VTand VF and unnecessary electrical stimulation therapies even whenpatient posture changes cause changes in QRS amplitude and morphology.

Sensing circuit 86 includes a second sensing channel 85 that receives asecond cardiac electrical signal from a second sensing vector, forexample from a vector that includes a pace/sense electrode 28 or 30paired with housing 15. Second sensing channel 85 may be selectivelycoupled to other sensing electrode vectors, which may form a relativelylong bipole having an inter-electrode distance or spacing that isgreater than the sensing electrode vector coupled to first sensingchannel 83 in some examples. As described below, the second cardiacelectrical signal received by second sensing channel 85 via a longbipole may be used by control circuit 80 for morphology analysis and fordetermining cardiac signal segment features for use in SVTdiscrimination. In other examples, any vector selected from theavailable electrodes, e.g., electrodes 24, 26, 28, 30 and/or housing 15,may be included in a sensing electrode vector coupled to second sensingchannel 85. The sensing electrode vectors coupled to first sensingchannel 83 and second sensing channel 85 are typically different sensingelectrode vectors, which may have no common electrodes or only onecommon electrode but not both.

In the illustrative example shown in FIG. 4, the electrical signalsdeveloped across a first sensing electrode vector are received bysensing channel 83 and electrical signals developed across a secondsensing electrode vector are received by sensing channel 85. The cardiacelectrical signals are provided as differential input signals to thepre-filter and pre-amplifiers 62 and 72, respectively, of first sensingchannel 83 and second sensing channel 85. Non-physiological highfrequency and DC signals may be filtered by a low pass or bandpassfilter included in each of pre-filter and pre-amplifiers 62 and 72, andhigh voltage signals may be removed by protection diodes included inpre-filter and pre-amplifiers 62 and 72. Pre-filter and pre-amplifiers62 and 72 may amplify the pre-filtered signal by a gain of between 10and 100, and in one example a gain of 17.5, and may convert thedifferential signal to a single-ended output signal passed toanalog-to-digital converter (ADC) 63 in first sensing channel 83 and toADC 73 in second sensing channel 85. Pre-filters and amplifiers 62 and72 may provide anti-alias filtering and noise reduction prior todigitization.

ADC 63 and ADC 73, respectively, convert the first cardiac electricalsignal from an analog signal to a first digital bit stream and thesecond cardiac electrical signal to a second digital bit stream. In oneexample, ADC 63 and ADC 73 may be sigma-delta converters (SDC), butother types of ADCs may be used. In some examples, the outputs of ADC 63and ADC 73 may be provided to decimators (not shown), which function asdigital low-pass filters that increase the resolution and reduce thesampling rate of the respective first and second cardiac electricalsignals.

In first sensing channel 83, the digital output of ADC 63 is passed tofilter 64 which may be a digital bandpass filter having a bandpass ofapproximately 10 Hz to 30 Hz for passing cardiac electrical signals suchas R-waves typically occurring in this frequency range. The bandpassfiltered signal is passed from filter 64 to rectifier 65 then to R-wavedetector 66. In some examples, the filtered, digitized cardiacelectrical signal from sensing channel 83, e.g., output of filter 64 orrectifier 65, may be stored in memory 82 for signal processing bycontrol circuit 80 for use in detecting and discriminatingtachyarrhythmia episodes.

R-wave detector 66 may include an auto-adjusting sense amplifier,comparator and/or other detection circuitry that compares the filteredand rectified first cardiac electrical signal to an R-wave sensingthreshold in real time and produces an R-wave sensed event signal 68when the cardiac electrical signal crosses the R-wave sensing thresholdoutside of a post-sense blanking period.

The R-wave sensing threshold, controlled by sensing circuit 86 and/orcontrol circuit 80, may be a multi-level sensing threshold as disclosedin pending U.S. patent application Ser. No. 15/142,171 (Cao, et al.,filed on Apr. 29, 2016), incorporated herein by reference in itsentirety. Briefly, the multi-level sensing threshold may have a startingsensing threshold value held for a time interval, which may be equal toa tachycardia detection interval or expected R-wave to T-wave interval,then drops to a second sensing threshold value held until a drop timeinterval expires, which may be 1 to 2 seconds long. The sensingthreshold drops to a minimum sensing threshold, which may correspond toa programmed sensitivity, after the drop time interval. In otherexamples, the R-wave sensing threshold used by R-wave detector 66 may beset to a starting value based on the most-recently sensed R-wave peakamplitude and decay linearly or exponentially over time until reaching aminimum sensing threshold. The techniques described herein are notlimited to a specific behavior of the sensing threshold. Instead, otherdecaying, step-wise adjusted or other automatically adjusted sensingthresholds may be utilized.

The second cardiac electrical signal, digitized by ADC 73 of sensingchannel 85, may be passed to filter 74 for bandpass filtering. In someexamples, filter 74 is a wideband filter for passing frequencies from 1to 30 Hz or higher. In some examples, sensing channel 85 includes notchfilter 76. Notch filter 76 may be implemented in firmware or hardwareand to attenuate 50 Hz or 60 Hz electrical noise in the second cardiacelectrical signal. Cardiac electrical signals acquired usingextra-cardiovascular electrodes may be more susceptible to 50 to 60 Hzelectrical noise than transvenous or intra-cardiac electrodes, musclenoise and other EMI, electrical noise or artifacts. As such, notchfilter 76 may be provided to significantly attenuate the magnitude ofsignals in the range of 50-60 Hz with minimum attenuation of signals inthe range of approximately 1-30 Hz, corresponding to typical cardiacelectrical signal frequencies.

The output signal 78 of notch filter 76 may be passed from sensingcircuit 86 to memory 82 under the control of control circuit 80 forstoring segments of the second cardiac electrical signal 78 in temporarybuffers of memory 82. For example, timing circuit 90 of control circuit80 may set a time interval or number of sample points relative to anR-wave sensed event signal 68 received from first sensing channel 83,over which the second cardiac electrical signal 78 is stored in memory82. The buffered, second cardiac electrical signal segment may beanalyzed by control circuit 80 on a triggered, as needed basis, e.g., asdescribed in conjunction with FIG. 13, for determining cardiac signalsegment features for discriminating SVT and withholding aninterval-based VT or VF detection, even when other R-wave morphologyanalysis meets VT/VF detection criteria.

Notch filter 76 may be implemented as a digital filter for real-timefiltering performed by firmware as part of sensing channel 85 or bycontrol circuit 80 for filtering the buffered digital output of filter74. In some examples, the output of filter 74 of sensing channel 85 maybe stored in memory 82 in time segments defined relative to an R-wavesensed event signal 68 prior to filtering by notch filter 76. Whencontrol circuit 80 is triggered to buffer and analyze segments of thesecond cardiac electrical signal, for example as described inconjunction with FIG. 13, the notch filter 76 may be applied to thesecond cardiac electrical before morphology analysis and determinationof cardiac signal segment features used for SVT discrimination.

The configuration of sensing channels 83 and 85 shown in FIG. 4 isillustrative in nature and should not be considered limiting of thetechniques described herein. The sensing channels 83 and 85 of sensingcircuit 86 may include more or fewer components than illustrated anddescribed in FIG. 4. First sensing channel 83 may be configured todetect R-waves from a first cardiac electrical signal in real time,e.g., in hardware implemented components, based on crossings of anR-wave sensing threshold by the first cardiac electrical signal, andsecond sensing channel 85 may be configured to provide a second cardiacelectrical signal for storage in memory 82 for processing and analysisby control circuit 80 for determining if the signal waveform morphologycorresponding to a sensed R-wave in the first sensing channel isindicative of VT or VF or if the signal waveform features support an SVTdetection and withholding of VT or VF detection. In other examples, bothsensing channels 83 and 85 may be capable of sensing R-waves in realtime and/or both channels 83 and 85 may provide a digitized cardiacsignal for buffering in memory 82 for morphological signal analysisduring VT/VF detection algorithms.

FIG. 5 is a flow chart 100 of a method performed by ICD 14 fordiscriminating SVT from a ventricular tachyarrhythmia according to oneexample. The flow chart 100 provides a technique for discriminating SVTfrom VT/VF, even when certain VT/VF morphology criteria are met, toaccount for QRS morphology changes that may occur due to changes inpatient posture in an extra-vascular ICD system. The techniques for SVTdiscrimination may be usefully practiced in other ICDs or other medicaldevice systems as well and are not necessarily limited to implantable,extra-cardiovascular systems. The techniques of flow chart 100 may beimplemented in conjunction with a variety of VT/VF detection algorithmsfor causing VT/VF detection to be withheld when SVT discriminationcriteria are satisfied, even though other VT/VF detection criteria maybe satisfied, such as RRI-based detection criteria and/or R-wavemorphology-based VT/VF detection criteria.

At block 102, control circuit 80 establishes an SVT morphology template.The morphology template may be established according to techniquesdisclosed in the above-incorporated U.S. Pat. No. 6,393,316 (Gillberg,et al.), and as generally described below in conjunction with FIG. 6.The SVT morphology template represents the expected R-wave morphologyduring a supraventricular rhythm, which may be a sinus rhythm or anatrial tachyarrhythmia that is conducted to the ventricles. Whilereferred to herein as an “SVT” template, the template may be acquiredduring a slow, non-paced ventricular rhythm to represent a normal QRSwaveform arising from the sinus node and is not necessarily acquiredduring supraventricular tachycardia. In other examples, the SVT templatemay be acquired during sinus tachycardia, for example during patientexercise.

At block 104, control circuit 80 compares the SVT template to themorphology of waveforms of the cardiac electrical signal received fromsensing circuit 86 corresponding to sensed R-waves. This comparison maybe made on a continuous beat-by-beat basis or only when other conditionsare met, such as an increase in heart rate. This morphology comparisonmay be performed using a wavelet transform technique as generallydisclosed in the above-incorporated '316 patent. The comparisondetermines a morphology match score that is a measure of the correlationbetween the SVT morphology template and the unknown cardiac electricalsignal waveform which may be stored in memory in response to a sensedR-wave. For example, a Haar transform or other wavelet transformtechnique may generate a set of wavelet coefficients for the signalwaveform. The wavelet coefficients may have predetermined weightingsrepresentative of the amplitudes of the frequency components of a signalwaveform. These wavelet coefficients may be compared to waveletcoefficients determined from the SVT template and the morphology matchscore may represent the correlation between the wavelet coefficients ofthe SVT template and the unknown signal waveform. A match scorethreshold may be defined, below which the unknown cardiac signalwaveform is not considered to be an R-wave corresponding to ansupraventricular rhythm and above which the waveform is considered to bean R-wave of a supraventricular rhythm. In one example, the morphologymatch score may range from 1 to 100, and the morphology match score maybe set to 60, 70 or another predetermined value.

At block 104, control circuit 80 determines if morphology criteria fordetecting VT/VF are met based on morphology comparisons between the SVTtemplate and unknown cardiac signal waveforms received by controlcircuit 80 from sensing circuit 86 and corresponding to R-wave sensedevent signals. VT/VF morphology criteria may be defined for determiningif the cardiac signal is likely to represent a rhythm originating in theventricles. In one example, the VT/VF morphology criteria may requirethat a predetermined percentage or ratio of R-wave sensed event signalsbe classified as non-SVT beats based on morphology analysis of cardiacsignal waveforms corresponding to each R-wave sensed event signal. AnR-wave sensed event signal may be classified as either an SVT beat or anon-SVT beat based on the most recent Y morphology matching scores ofthe unknown cardiac signal waveforms.

For example, if at least 6 out of 8 of the most recently acquiredcardiac signal waveforms, each corresponding to an R-wave sensed eventsignal, do not match the SVT morphology template, based on a morphologymatch score being less than the match threshold, the latest one of theR-wave sensed event signals of the group of 8 sensed event signals isclassified as a potential VT/VF beat. When a threshold number of R-wavesensed event signals are classified as potential VT/VF beats, VT/VFmorphology criteria may be satisfied at block 104. The threshold numberof R-wave sensed events being classified as potential VT/VF beatsrequired to satisfy the VT/VF morphology criteria may be one or more andmay be dynamically adjusted by control circuit 80, e.g., as described inconjunction with FIG. 10.

Each of the eight (or other predetermined number of) cardiac signalwaveforms used to classify a given R-wave sensed event signal as apotential VT/VF beat may be obtained from a cardiac electrical signalsegment that is buffered in memory 82 from the second cardiac electricalsignal received from the second sensing channel 85 of sensing circuit86. These cardiac signal segments may be acquired over a time intervalset based on the timing of an R-wave sensed event signal produced by thefirst sensing channel 83. As described below, the buffering of thesecardiac signal segments, each corresponding to an R-wave sensed eventsignal produced by the first sensing channel 83, may be triggered atblock 104 when control circuit 80 determines that specified conditionsare met, e.g., when evidence of a fast ventricular rate has beendetected based on a predetermined number of VT or VF intervals beingcounted.

If VT/VF morphology criteria are not met at block 104, control circuit80 continues to monitor the cardiac electrical signal for evidence ofVT/VF morphology by analyzing buffered cardiac signal segmentscorresponding to R-wave sensed event signals. If the VT/VF morphologycriteria are satisfied at block 104, control circuit 80 determines ifcriteria for enabling SVT discrimination are satisfied at block 106. Inone example, criteria for enabling SVT discrimination requires that theVT/VF morphology criteria are met at block 104 and that at least X outof Y most recent cardiac signal waveforms match the SVT morphologytemplate with a matching score that is greater than a second matchthreshold. The second match threshold is less than the first, morphologymatch threshold applied at block 104 for classifying R-wave sensedevents as potential VT/VF. In one example, if the first match thresholdapplied at block 104 is 60, the second match threshold applied at block106 is 20.

To illustrate, if cardiac signal waveforms are required to have amatching score that is less than a match threshold of 60 according toVT/VF morphology criteria at block 104, SVT discrimination may beenabled at block 106 when VT/VF morphology criteria are met and at least6 out of 8 most recent cardiac signal waveforms corresponding to R-wavesensed event signals match the SVT morphology template with a matchingscore of at least 20. SVT discrimination may be enabled when themorphology matching scores of the most recent group of cardiac signalwaveforms are not highly correlated with the SVT template (less thanfirst match threshold), suggesting a ventricular tachyarrhythmia, butare greater than a second, lower match threshold. The QRS morphologyduring a supraventricular rhythm may change compared to the SVTtemplate, e.g., due to postural changes of the patient, causing themorphology matching scores to be less than the first match threshold. Inorder to avoid a false VT/VF detection due to a change in QRS morphologyduring an SVT rhythm, SVT discrimination is enabled at block 106 if themorphology match scores that led to the VT/VF morphology criteria beingmet are at least greater than the second match threshold.

On the other hand, very low morphology match scores that are less thanthe second match threshold indicate very low correlation to the SVTtemplate. If fewer than 6 out of 8 (or other predetermined percentage),of the most recent cardiac signal segments have morphology match scoresless than the second threshold, a likelihood of a VT/VF rhythm exists.In this case, SVT discrimination is not enabled. If criteria forenabling the SVT discrimination are not satisfied at block 106, theVT/VF morphology criteria satisfied at block 104 may be determined to beevidence of a tachyarrhythmia that is ventricular in origin and does notrequire further discrimination. Control circuit 80 advances toward VT/VFdetection at block 120 without performing additional morphology analysisof the cardiac signal for discriminating SVT.

If criteria for enabling SVT discrimination are satisfied at block 106,e.g., if at least X of Y morphology match scores are greater than thesecond, lower morphology match threshold (but less than the first,higher match threshold), control circuit 80 begins determining multiplefeatures of each of the buffered cardiac signal segments at block 108for use in discriminating SVT from VT/VF. Determination of cardiacsignal segment features is described below in conjunction with FIGS.7-9. At block 110, a first portion of the cardiac signal segmentfeatures determined from each of one of a group of Y cardiac signalsegments are compared to monomorphic waveform criteria. At block 112, asecond portion of the cardiac signal segment features determined fromeach of the Y buffered signal segments are compared to SVT beatcriteria.

If monomorphic waveform criteria and SVT beat criteria are satisfied atblocks 110 and 112, respectively, the rhythm may be identified as an SVTrhythm at block 114. A VT/VF detection indicated by the VT/VF morphologycriteria being satisfied at block 104 is withheld at block 116, andVT/VF therapy is subsequently not delivered. At block 118, controlcircuit 80 may adjust the VT/VF morphology detection criteria to delayVT/VF detection if cardiac signal segments continue to fail to match theSVT template based on the first match threshold at block 104. Forexample, the number of R-wave sensed event signals required to beclassified as potential VT/VF beats in order to satisfy the VT/VFmorphology criteria at block 104 may be reset or adjusted at block 118,as described below in conjunction with FIG. 10.

In some examples, both the monomorphic waveform criteria and the SVTbeat criteria are required to be satisfied at blocks 110 and 112 inorder to detect SVT and withhold VT/VF detection and therapy. If eitherone of the monomorphic waveform criteria or the SVT beat criteria arenot satisfied, SVT is not detected. Control circuit 80 advances to block120, and, if VT or VF detection criteria are satisfied at block 120, VTor VF is detected at block 122. For example, if the VT/VF morphologycriteria are satisfied at block 104, and an SVT detection is not made(“no” branch of block 112), control circuit 80 may detect VT or VF atblock 122 if the respective VT or VF NID is reached at block 120. Ifinterval-based VT/VF detection criteria are not satisfied at block 120,control circuit continues to monitor the cardiac electrical signal byreturning to block 104.

If both the VT/VF morphology criteria and the VT/VF interval criteriaare satisfied at respective blocks 104 and 120, and SVT beat criteriaare not met at block 112 (or the SVT discriminator is not enabled atblock 106), VT or VF is detected at block 122. An electrical stimulationtherapy may be scheduled and delivered at block 122. The electricalstimulation therapy may include ATP and/or a CV/DF shock delivered bytherapy delivery circuit 84 according to a programmed therapy protocolfor the detected VT or VF rhythm.

The blocks shown in FIG. 5 may be performed in a different order thanthe particular order shown in FIG. 5. In other examples, morphologymatch scores and cardiac signal segment features may be determined atblocks 104 and 108 in response to a threshold number of RRIs fallinginto a VT or VF interval zone, indicating the possible onset of a fastheart rate. Control circuit 80 may determine if the monomorphic waveformcriteria and the SVT beat criteria are met at blocks 110 and 112,respectively, in response to an NID being reached to determine if VT/VFdetection should be made or withheld based on SVT discriminationcriteria being met. In still other examples, the morphology match scoresand/or cardiac signal segment features may be determined on abeat-by-beat basis such that, once an NID is reached, the VT or VFdetection may be made as long as the monomorphic waveform and SVT beatcriteria are unmet for at least the most recent group of Y cardiacsignal segments.

FIG. 6 is a flow chart 150 of a method that may be performed by controlcircuit 80 for establishing the SVT morphology template at block 102 ofFIG. 5. At block 152, control circuit 80 may acquire a predeterminednumber of R-wave signals (or QRS complexes) from a cardiac electricalsignal received from sensing circuit 86 during a known supraventricularrhythm. For example, a sinus rhythm may be confirmed manually by a userusing external device 40 or automatically by detecting a normal heartrate (e.g., less than a tachyarrhythmia rate associated with VT/VFdetection intervals) and/or regular, stable R-wave signals. For example,three or more R-wave signals may be acquired at block 152. These R-wavesignals may be notch-filtered signals received from the second sensingchannel 85, each corresponding to an R-wave sensed event signal receivedfrom the first sensing channel 83. The notch-filtered R-wave signalsegments may be aligned in time relative to the time of thecorresponding R-wave sensed event signal. In other examples, a differentreference time point or sample number may be used to align the R-wavesignal segments such as a maximum peak or other fiducial point. Thenotch-filtered R-wave signals may then be ensemble averaged to obtain anaveraged R-wave signal to establish an SVT morphology template for usein determining if VT/VF morphology criteria are met at block 104 of FIG.5. In other examples, the template may be generated from a single R-wavesignal acquired during sinus rhythm.

At block 154, wavelet transform coefficients are determined from theaveraged R-wave signal. Determination of wavelet transform coefficientsmay be performed according to the above-incorporated '316 patent(Gillberg, et al.). The digitized averaged R-wave signal and/or thewavelet transform coefficients may be stored in memory 82 as the SVTmorphology template.

At block 156, template features are determined from the averaged R-wavesignal. These template features are used when SVT discrimination isperformed, e.g., if SVT discrimination criteria are met at block 106 ofFIG. 5. In some examples, the template features determined at block 156are used at block 112 of FIG. 5 for comparison to a portion of thesignal waveform features determined from each cardiac signal segment ofa group of Y cardiac signal segments. The template features may becompared to signal waveform features determined from cardiac signalsegments when SVT discrimination is enabled for determining whether SVTbeat criteria are met at block 112. The template features determined atblock 156 may or may not be used in determining whether monomorphicwaveform criteria are met at block 110 of FIG. 5. As described inconjunction with FIG. 11 below, a portion of the signal waveformfeatures determined from each one of a group of Y cardiac signalsegments may be compared to each other for determining if monomorphicwaveform criteria are satisfied without comparing these signal waveformfeatures to SVT template features.

The template features determined at block 156 may include a polaritypattern, a peak time interval, and an averaged signal width. Exampletechniques for determining these features are described in conjunctionwith FIGS. 7 and 8. The template features are stored in memory 82 foruse in determining if SVT beat criteria are satisfied when SVTdiscrimination has been enabled by control circuit 80.

FIG. 7 is a diagram of one example of a notch-filtered cardiac signalsegment 160 from which cardiac signal segment features are determined atblock 108 of FIG. 5 when SVT discrimination is enabled. Cardiac signalsegment 160 may include a predetermined number of sample points beforeand after an R-wave sensed event signal 162 produced by sensing circuit86, corresponding to the time of a crossing of an R-wave sensingthreshold by a cardiac electrical signal. In one example, cardiac signalsegment 160 includes 48 sample points with the R-wave sensed eventsignal 162 occurring at the twenty-fourth sample point.

The R-wave sensed event signal 162 may be produced when cardiac signalsegment 160 crosses an R-wave sensing threshold but may be produced whena different cardiac electrical signal, e.g., from a different one ofsensing channels 83 and 85, crosses the R-wave sensing threshold. Forinstance, the first sensing channel 83 may produce R-wave sensed eventsignal 162 in response to the first cardiac electrical signal receivedby first sensing channel 83 crossing an R-wave sensing threshold. Thecardiac signal segment 160 may be buffered from the second cardiacelectrical signal received by control circuit 80 from the second sensingchannel 85. The R-wave sensed event signal 162 from the first sensingchannel 83 is used as a timing marker for selecting the beginning andending sample points stored from the second cardiac electrical signalfor buffering cardiac signal segment 160. In this way, the first sensingchannel 83 may be used for sensing R-waves, and the second sensingchannel 85 may be used for acquiring cardiac signal segments from adifferent sensing vector. Each cardiac signal segment corresponds to anR-wave sensed event signal 162. Cardiac signal segment features aredetermined from each cardiac signal segment, such as segment 160, forSVT discrimination.

One feature determined from cardiac signal segment 160 may be itspolarity pattern. An R-wave signal may have a biphasic polarity havingboth a pronounced positive and pronounced negative peak. At other times,an R-wave signal may have a monophasic polarity characterized by asingle dominant peak, either positive or negative. Control circuit 80may be configured to identify and discriminate between four polaritypatterns: biphasic having a positive peak followed by a negative peak;biphasic having a negative peak followed by a positive peak; monophasichaving a positive dominant peak, or monophasic having a negativedominant peak. Polarity pattern values may be assigned to each of thepossible polarity patterns for buffering in memory 82 for apredetermined number of cardiac signal segments analyzed during SVTdiscrimination. For instance, the four polarity patterns listed abovemay be assigned respective values of 1 through 4. In other examples,polarity patterns identified by control circuit 80 may not be limited tothe four patterns listed above; control circuit 80 may be configured toidentify fewer, additional or different polarity patterns than the fourlisted here. Polarity patterns that are identified may be tailored to anindividual patient or based on implant locations of sensing electrodevectors. For example, an R-wave signal may include more than twopronounced peaks in a tri-phasic signal or a signal having a pronouncedsplit positive peak and a pronounced negative peak or vice versa.

Control circuit 80 may determine the polarity pattern of cardiac signalsegment 160 by determining the maximum positive amplitude 164 of themaximum peak 163 and the maximum negative amplitude 166 of the minimumpeak 165. The greatest absolute value of the maximum positive andnegative amplitudes 164 and 166, respectively, is identified and may beused by control circuit 80 to set a polarity pattern amplitudethreshold. If the absolute values of both of the maximum positiveamplitude 164 and maximum negative amplitude 166 are greater than thepolarity pattern amplitude threshold, the cardiac signal segment 160 isdetermined to have a biphasic polarity pattern. If only one of themaximum amplitudes 164 or 166 is greater than the polarity patternamplitude threshold, the cardiac signal segment is determined to have amonophasic polarity pattern.

In an illustrative example, the polarity pattern amplitude threshold isset to be 25% of the largest one of the maximum positive and negativeamplitudes 164 and 166. In the particular example shown in FIG. 7,maximum positive amplitude 164 is greater in absolute value than maximumnegative amplitude 166. Control circuit 80 therefore uses the maximumpositive amplitude 164 to set the polarity pattern amplitude thresholdas 25% of maximum positive amplitude 164. The absolute value of themaximum negative amplitude 166 is compared to the polarity patternamplitude threshold. Since it is greater than the polarity patternamplitude threshold, i.e., greater than 25% of the maximum positiveamplitude 164 in this example, the cardiac signal segment 160 isdetermined to have a biphasic polarity pattern.

The control circuit 80 may further determine that the positive peak 163occurs earlier in time than the negative peak 165 yielding a polaritypattern of biphasic, positive peak first. The sample point numbers ofmaximum peak 163 and minimum peak 165 may be compared to determine ifthe biphasic pattern is positive peak first or negative peak first. Thesample points in cardiac signal segment 160 may be numberedconsecutively from beginning to end, e.g., from 1 to 48 when 48 samplepoints are included in cardiac signal segment 160. A lower sample pointnumber of maximum peak 163 and a higher sample point number of minimumpeak 165 indicate a positive peak first polarity pattern. Controlcircuit 80 may store a value in memory 82 indicating that the polaritypattern of cardiac signal segment 160 is biphasic, positive peak first.

A second SVT discrimination feature of the cardiac signal segment 160may be determined as a peak time interval 168. In the example of abiphasic polarity pattern, the peak time interval 168 may be determinedas the time interval between the maximum peak 163 and the minimum peak165. This peak time interval 168 may be determined and stored in memory82 for cardiac signal segment 160 as the difference between therespective sample point numbers of the maximum positive peak 163 and theminimum negative peak 165.

FIG. 8 is a diagram of an example cardiac signal segment 170 having amonophasic polarity pattern. The maximum positive amplitude 174 is usedby control circuit 80 to set the polarity pattern amplitude thresholdbecause it is greater than the maximum negative amplitude 176. Theabsolute value of the maximum negative amplitude 176 of minimum peak 175is less than the polarity pattern threshold, which may be set toone-fourth of the maximum positive amplitude 174. The maximum peak 173is therefore the only dominant peak. Control circuit 80 identifiescardiac signal segment 170 as having a monophasic, positive polaritypattern and stores a polarity pattern value in memory 82 indicating thispolarity pattern for cardiac signal segment 170.

When the polarity pattern is determined to be monophasic, the controlcircuit 80 may determine the peak time interval 178 using a differentmethod than the method used to determine the peak time interval 168 of abiphasic polarity pattern signal 160 as shown in FIG. 7. The peak timeinterval 178 of a monophasic signal may be determined as the timeinterval, or sample point number difference, between the R-wave sensedevent signal 172 and the dominant peak, maximum peak 173 in thisexample.

A third SVT discrimination feature that may be determined from cardiacsignal segments 160 and 170 of FIGS. 7 and 8, in addition to the peaktime interval and the polarity pattern, may be a normalized signalwidth. The cardiac signal segment 160 or 170 may be rectified and allsample point amplitudes may be summed to obtain an area defined by thesignal segment 160 or 170. The area of the signal 160 or 170 may bedivided by the largest absolute value of either the maximum peakamplitude 164 or 174 or minimum peak amplitude 166 or 176 of therespective signal segment to obtain the normalized signal width. Each ofthese three features, namely polarity pattern, peak time interval andnormalized signal width, are stored for each buffered cardiac signalsegment during SVT discrimination. These three cardiac signal segmentfeatures may be used for determining if a sensed R-wave satisfies SVTbeat criteria at block 112 of FIG. 5 and as further described inconjunction with FIG. 12 below.

The polarity pattern, peak time interval, and normalized signal widthmay be determined from the SVT template in a similar manner at block 156of FIG. 6. In this way, cardiac signal segment features determined fromsignal segments acquired during an unknown heart rhythm may be comparedto analogous SVT template features when SVT discrimination is enabledfor determining if SVT beat criteria are satisfied.

FIG. 9 is a schematic diagram of a notch-filtered cardiac signal segment180 for use in determining if monomorphic waveform criteria aresatisfied at block 110 of FIG. 5. In addition to the features determinedfrom cardiac signal segments for determining if SVT beat criteria aresatisfied, features are determined from the buffered, cardiac signalsegments for applying monomorphic waveform criteria when SVTdiscrimination is enabled. One feature determined by control circuit 80for use in confirming whether monomorphic waveform criteria are met maybe the maximum peak sample number. As described above, the sample pointsincluded in a buffered cardiac signal segment 180 may be numberedconsecutively from beginning to end, e.g., from 1 to 48, with thecorresponding R-wave sensed event signal 182 at sample point number 24.The polarity of a dominate peak of the SVT template may be determined atblock 156 of FIG. 6. The maximum peak sample number of signal segment180 is determined as the sample point number having a maximum absoluteamplitude and the same polarity as the dominate peak of the SVTtemplate.

For example, if the SVT template is identified to have a dominant peakthat has a positive polarity, i.e., the absolute maximum amplitude ofthe SVT template is positive, the sample point number of the maximumpeak 183 is identified as the maximum peak sample number. If, however,the SVT template has a maximum absolute amplitude corresponding to aminimum (negative) peak, the sample point number of the minimum peak 185is determined as the maximum peak sample number. Control circuit 80 maysearch for a maximum value of all positive values of cardiac signalsegment 180 if the dominant peak of the SVT template is positive orsearch for a minimum of all negative values of the cardiac signalsegment 180 if the dominant peak of SVT template is negative. The samplepoint number of the respective maximum or minimum is stored as themaximum peak sample number for the corresponding cardiac signal segment180.

In addition to determining the maximum peak sample number, controlcircuit 80 may determine the maximum peak amplitude 184 or 186 of themaximum peak 183 or 185 that has a polarity matching the dominant peakof the SVT template. In some cases, the maximum peak sample number andmaximum peak amplitude determined from cardiac signal segment 180 as SVTdiscrimination features may or may not correspond to the actual absolutemaximum peak amplitude of the segment 180 when the actual absolutemaximum peak of segment 180 has a different polarity than the polarityof the greatest absolute amplitude of the SVT template.

A third SVT discrimination feature determined from cardiac signalsegment 180 for use in determining if monomorphic waveform criteria aremet may be the RRI from the R-wave sensed event signal 182 to the mostrecent preceding R-wave sensed event signal. The maximum peak samplenumber and maximum peak amplitude having a polarity matching thepolarity of the maximum absolute amplitude of the SVT template and theRRI may all be stored as features of segment 180 for determining ifmonomorphic waveform criteria are met when SVT discrimination isenabled.

In other examples, the absolute maximum peak of the cardiac signalsegment 180 may be identified and its amplitude and sample number may bestored as features of cardiac signal segment 180 regardless of polarity.The maximum peak 183 has a greater amplitude 184 than the amplitude 186of minimum peak 185. The maximum peak amplitude 184 may be stored asfeature of cardiac signal segment 180, along with the sample number ofmaximum peak 183. If the minimum peak 185 has a larger negative peakamplitude 186 (absolute value) than positive peak amplitude 184, theabsolute value of the negative peak amplitude 186 and the sample numberof minimum peak 185 may be stored as features of cardiac signal segment180.

FIG. 10 is a flow chart 200 of a method for discriminating SVT fromVT/VF and for adjusting VT/VF morphology criteria according to anotherexample. At block 202, control circuit 80 sets a VT/VF morphology countto an initial value. Control circuit 80 may include a counter forcounting the number of sensed R-waves classified as a potential VT/VFbeat based on at least X of Y most recent cardiac signal segments havinga morphology matching score less than a first match threshold. In theillustrative examples presented herein, the counter is initially set toa value of zero so that only a single group of Y cardiac signal segmentsresulting in the most recent R-wave sensed event signal being classifiedas a potential VT/VF beat results in VT/VF morphology criteria beingmet.

At block 204, the morphology match scores for Y cardiac signal segmentsthat are buffered in memory 82 are determined. The Y morphology matchscores are compared to the match threshold at block 206. These Ymorphology match scores may be compared to the match threshold on abeat-by-beat basis so that each R-wave sensed event signal may beclassified as a potential VT/VF beat if at least X of the most recent Ymorphology match scores stored in a rolling buffer of memory 82 are lessthan the first match threshold. If fewer than X of the Y morphologymatch scores are less than the match threshold, “no” branch of block206, the most recently sensed R-wave is not classified as a potentialVT/VF beat.

In response to not classifying the most recently sensed R-wave as apotential VT/VF beat, control circuit adjusts the VT/VF counter to delaydetection of VT or VF. For example, the counter value initialized tozero at block 202 may be set to a non-zero value, e.g., 10, at block 207when at least X of Y match scores are not less than the match thresholdat block 206. This means that more than Y-X morphology match scores aregreater than the match threshold and may represent heartbeats thatoriginated in the upper heart chambers, indicating an SVT rhythm. Thisevidence of an SVT rhythm warrants delaying VT or VF detection. As such,the VT/VF morphology counter value is set to a non-zero value at block207 to delay a VT/VF detection. For example, each time fewer than X of Ycardiac signal segments have a morphology matching score that is lessthan the first match threshold, the currently sensed R-wave is notclassified as a potential VT/VF beat, and the VT/VF counter is set to 10at block 207. The VT/VF counter may be required to count down from anon-zero value back to zero before VT or VF can be detected, therebydelaying a VT or VF detection in the presence of morphology-based SVTrhythm evidence. In order to reach a count of zero, 10 consecutivelysensed R-waves may be required to be classified as a potential VT/VFbeat based on at least X of Y most recent cardiac signal segments havinga morphology match score greater than the first match threshold. Atblock 230, control circuit 80 advances to the next R-wave signal fordetermining the next morphology match score in a moving window of YR-wave signals.

Using the previous example, if 6 out of 8 cardiac signal segments havemorphology match scores less than a first match threshold of 60, thecontrol circuit 80 classifies the most recently sensed R-wave as apotential VT/VF beat at block 208. The VT/VF morphology counter value isdecreased by one at block 209. If the counter value is at zero, e.g.,still at an initialized zero value after the evaluation of the firstgroup of Y morphology match scores, no adjustment is needed at block209. Otherwise, if the counter value had been previously adjusted atblock 207, the counter value is decreased by one. If the counter valueis not equal to zero at block 210, control circuit 80 advances to block230 to determine the next morphology match score for the next bufferedcardiac signal segment corresponding to the next R-wave sensed eventsignal. The oldest buffered cardiac signal segment may be dropped, alongwith its match score, so that a moving window of Y cardiac signalsegments is advanced forward by one sensed R-wave. This process ofdetermining the next morphology match score and whether at least X of Ymorphology match scores are less than the morphology match threshold(block 206) continues until the VT/VF morphology counter reaches a valueof zero at block 210.

If the VT/VF morphology counter value is at zero at block 210, controlcircuit 80 advances to block 212 to determine if criteria for enablingSVT discrimination are satisfied. A VT/VF morphology counter value ofzero indicates that at least one R-wave sensed event signal isclassified as a potential VT/VF beat at block 208 based on X of Y matchscores being less than the first match threshold. When the VT/VFmorphology counter reaches zero the VT/VF morphology criteria may besatisfied, supporting a VT or VF detection if other VT/VF detectioncriteria are satisfied.

However, postural changes and other factors may influence the cardiacelectrical signal received by sensing circuit 86. As a result,relatively low morphology match scores may occur, leading to the VT/VFmorphology criteria being satisfied at block 210, at times when theheart rhythm is actually a supraventricular rhythm. As such, beforesupporting a VT/VF detection based on the VT/VF morphology criteriabeing satisfied according to a counter value of zero at block 210,control circuit 80 determines if SVT discrimination criteria are met atblock 212.

In one example, control circuit 80 enables SVT discrimination at block214 if at least X of the most recent Y cardiac signal segments have amorphology match score that is greater than a second match threshold atblock 212, which may be referred to as an SVT discrimination matchthreshold. The SVT discrimination match threshold applied at block 212may be less than the first match threshold applied to morphology matchscores at block 206 for determining if VT/VF morphology criteria aresatisfied. In the example given above, a first match threshold of 60 isapplied at block 206. The SVT match threshold applied at block 212 maybet 20. The SVT discrimination criteria may require that X of Y cardiacsignal segments have a morphology match score of at least 20 in order toenable SVT discrimination at block 214.

If this requirement is not met, “no” branch of block 212, controlcircuit 80 may advance to block 216 to determine if VT/VF detectioncriteria are met without enabling SVT discrimination. If other VT/VFdetection criteria are satisfied, e.g., if a VT or VF interval counterreaches as respective NID, and the VT/VF morphology counter value is atzero as determined at block 210, control circuit 80 may detect VT or VFat block 222, without performing SVT discrimination.

If SVT discrimination criteria are met at block 212, however, controlcircuit 80 enables SVT discrimination at block 214. Control circuit 80enables SVT discrimination at block 214 by determining cardiac signalsegment features from each buffered cardiac signal segment as describedin conjunction with FIGS. 7-9. If VT/VF detection criteria are met atblock 216, e.g., if an NID is reached and the VT/VF morphology counterhas reached a count of zero at block 210, control circuit 80 determinesif SVT detection criteria are met at block 218.

At block 218, a first portion of the features determined from each ofthe currently buffered cardiac signal segments are used for determiningif the group of Y cardiac signal segments represent monomorphicwaveforms, and a second portion of the features determined from each ofthe Y cardiac signal segments are used to determine if the group of Ycardiac signal segments represent SVT beats. Methods for determiningwhether monomorphic waveform criteria and SVT beat criteria aresatisfied are described below in conjunction with FIGS. 11 and 12. Ifthe Y cardiac signal segments are determined to satisfy both monomorphicwaveform criteria and SVT beat criteria, SVT detection criteria aresatisfied at block 218. SVT is detected at block 224. VT/VF detection iswithheld at block 226, and no VT/VF therapy is delivered.

In response to the SVT detection criteria being met at block 218,control circuit 80 may adjust the VT/VF morphology counter to a non-zerovalue at block 228. In one example, the VT/VF morphology counter, whichhad reached zero at block 210, may be increased to a value of 5 oranother predetermined value, to delay VT/VF detection. Since the VT/VFmorphology counter is no longer at a value of zero, SVT discriminationmay be disabled at block 229. The VT/VF morphology counter may berequired to count back down to zero from the adjusted count value of 5before VT or VF can be detected. Control circuit 80 returns to block 230to advance the moving window of Y R-wave signals to the next sensedR-wave and determines the morphology matching score of the next bufferedcardiac signal segment.

If the SVT detection criteria are not met at block 218, and all otherVT/VF detection criteria are satisfied, e.g., a VT or VF NID is reachedwhen the VT/VF morphology counter is at a value of zero, VT or VF isdetected at block 222. Control circuit 80 may control therapy deliverymodule 84 to deliver therapy according to programmed therapy protocolsfor the detected VT or VF episode.

FIG. 11 is a flow chart 300 of a method that may be performed at block110 of FIG. 5 or block 218 of FIG. 10 for determining if monomorphicwaveform criteria are satisfied according to one example. After SVTdiscrimination is enabled at block 106 of FIG. 5 (or block 214 of FIG.10), control circuit 80 may be configured to determine features fromcardiac signal segments buffered in memory 82 corresponding torespective R-wave sensed event signals received from sensing circuit 86.As described above, the cardiac signal segments may be acquired from asecond cardiac electrical signal received by second sensing channel 85.The cardiac signal segments may include 48 sample points with the R-wavesensed event signal at sample point 24. The second cardiac electricalsignal may be notch filtered by second sensing channel 85 prior todetermining the features of the buffered cardiac signal segments.

As described in conjunction with the examples of FIGS. 7-9, sixdifferent features are determined from each cardiac signal segment.Those six features include three features determined from each one ofthe cardiac signal segments for use in determining if monomorphicwaveform criteria are met. The other three features determined from eachone of the cardiac signal segments are used for determining if SVT beatcriteria are met as described below in conjunction with FIG. 12. Thefeatures may be determined from each cardiac signal segment on abeat-by-beat basis as each signal segment is acquired and buffered inmemory 82. The determined features may be buffered in memory 82 for apredetermined number of signal segments corresponding to consecutivelysensed R-waves, e.g., 8 signal segments, in a first-in-first-out basis.In this way, control circuit 80 may determine if SVT is detected usingthe most recent Y buffered cardiac signal segment features if VT/VFdetection criteria become satisfied. The VT/VF detection may be withheldbased on an SVT detection.

The three features determined and stored for each one of the cardiacsignal segments for determining if monomorphic waveform criteria are metinclude the maximum peak amplitude, the maximum peak sample number andthe RRI as described in conjunction with FIG. 9 above. At block 302,control circuit 80 determines the maximum peak amplitude variabilityamong the Y maximum peak amplitudes determined and stored for the Ycardiac signal segments. The maximum peak amplitude variability may bedetermined by determining the largest maximum peak amplitude and thesmallest maximum peak amplitude among the buffered Y maximum peakamplitudes. The maximum peak amplitude variability may be determined atblock 302 as the difference between the largest and smallest maximumpeak amplitudes divided by the mean of the Y buffered maximum peakamplitudes.

At block 306 a maximum peak timing variability is determined using the Ymaximum peak sample numbers buffered for the Y cardiac signal segments.The maximum peak timing variability may be determined as the differencebetween the largest and the smallest maximum peak sample numbers storedfor the Y cardiac signal segments.

Control circuit 80 determines RRI variability at block 308. RRIvariability may be determined by subtracting the smallest RRI from thelargest RRI stored for the Y cardiac signal segments and dividing thedifference by a mean RRI determined from the Y RRIs. In one example, themean RRI is a trimmed mean determined by averaging the buffered RRIvalues after dropping the largest and smallest RRIs. In some examples,the RRI variability may be determined over more than Y cardiac signalsegments. For instance, the most recent 12 RRIs may be used fordetermining RRI variability at block 308. The trimmed mean may bedetermined by dropping the largest two RRIs and the smallest two RRIsand averaging the remaining 8 RRIs. The difference between the maximumand minimum RRIs of all 12 RRIs may be divided by the trimmed mean todetermine the RRI variability at block 308.

It is recognized that other techniques may be used to determinevariability in the maximum peak amplitude, the variability in themaximum peak timing, which is related to the timing of the R-wave sensedevent signal since the sample point numbering of the signal segment isbased on the timing of the R-wave sensed event signal (as shown in FIG.9), and the variability in RRIs. Once SVT discrimination is enabled, thecardiac signal segment features needed for applying monomorphic waveformcriteria may be determined so that the variability of the cardiac signalsegment features for the most recent Y cardiac signal segments may beperformed on a beat-by-beat basis.

At block 310, control circuit 80 compares each of these variabilitymetrics determined at blocks 302, 306 and 308 to respective thresholds.If each variability metric is less than its respective threshold,indicating relatively low variability in the maximum peak amplitude,maximum peak timing, and RRIs, the monomorphic waveform criteria aresatisfied as indicated at block 312. In one example, if the maximum peakamplitude variability is less than 60%, the maximum peak timingvariability is less than 8 sample points (for a sampling rate of 256Hz), and the RRI variability is less than 15%, monomorphic waveformcriteria are met. These examples are intended to be illustrative innature and not limiting; other variability thresholds may be used foridentifying monomorphic waveforms.

In some examples, all three variability metrics are required to be lessthan their respective thresholds, and, if not, the monomorphic waveformcriteria are not met as indicated at block 314. In other examples, atleast one or two of the variability metrics may be required to be lessthan their respective threshold in order for monomorphic waveformcriteria to be satisfied. In the analysis of FIG. 11, the three featuresof maximum peak amplitude, maximum peak timing, and RRI determined foreach of the buffered Y cardiac signal segments are compared to eachother for determining variability metrics and whether or not thecorresponding sensed R-waves are monomorphic. These features may not becompared to SVT template features in the example method of FIG. 11 fordetermining if the Y cardiac signal segments meet monomorphic waveformcriteria.

FIG. 12 is a flow chart 400 of a method for determining if SVT beatcriteria are met at block 112 of FIG. 5 or at block 218 of FIG. 10. Inthe examples presented in conjunction with FIGS. 7 and 8, the threefeatures determined from buffered cardiac signal segments fordetermining if SVT beat criteria are met are polarity pattern, peak timeinterval, and normalized signal width. At block 410, control circuit 80compares these three features determined from the most recent bufferedcardiac signal segment to the analogous three features of the SVTtemplate (determined and stored in memory 82 at block 156 of FIG. 6).

If the three features match the respective SVT template features withina predetermined threshold range, the most recently sensed R-wavecorresponding to the cardiac signal segment is counted as an SVT beat atblock 414. If the cardiac signal segment features do not match theanalogous SVT template features within a predetermined threshold range,the process advances to block 416 without counting the most recentlysensed R-wave as an SVT beat.

In one example, the determined polarity pattern of the cardiac signalsegment and the SVT template are required to be the same in order tomatch at block 412. For instance both the cardiac signal segment and theSVT template are biphasic, positive peak first; both are biphasic,negative peak first; both are monophasic, positive peak or both aremonophasic, negative peak in order to match. The peak time interval ofthe cardiac signal segment may be required to within 5 sample points ofthe peak time interval of the SVT template (for a sampling rate of 256Hz) in order for the peak time interval to match the SVT template peaktime interval. The normalized signal width may be required to be within30% or another percentage threshold of the SVT template normalizedsignal width. If each of these three feature comparisons to analogousSVT template features result in a match, the most recently sensed R-wavecorresponding to the cardiac signal segment from which the features werederived is counted as an SVT beat at block 414.

Control circuit 80 may include an SVT beat counter for counting each ofthe most recent Y cardiac signal segments having the three SVTdiscrimination features matching the SVT template features. If thecounter is equal to or greater than an SVT beat count threshold at block416, the SVT beat criteria are determined to be satisfied at block 422.If the SVT beat counter is not equal to or greater than the SVT beatcount threshold at block 416 (“no” branch), the SVT beat criteria forthe Y cardiac signal segments are determined to not be met at block 424.The SVT count threshold applied at block 416 may require that 3 out ofthe most recent 8 cardiac signal segments be counted as SVT beats. Inother examples, more or fewer than 3 out of 8 cardiac signal segmentsmay be required to be counted as SVT beats in order to satisfy the SVTbeat criteria at block 422.

As described above, if both the monomorphic waveform criteria and theSVT beat criteria are satisfied by the SVT discrimination featuresdetermined from the most recent Y cardiac signal segments, SVT detectioncriteria are met. In other examples, control circuit 80 may require boththe monomorphic waveform criteria and the SVT beat criteria be satisfiedby the SVT discrimination features determined from the most recent Ycardiac signal segments for more than group of Y cardiac signalsegments. For instance, the monomorphic waveform and SVT beat criteriamay both be required to be satisfied by the most recent Y cardiac signalsegments on three or more consecutively sensed R-waves. If SVT detectioncriteria are satisfied at the time that VT/VF detection criteria aremet, the VT/VF detection is withheld.

FIG. 13 is a flow chart 500 of a method for detecting ventriculartachyarrhythmias according to one example using the SVT discriminationtechniques disclosed herein. At block 502, a sensing electrode vector isselected by sensing circuit 86 for receiving a cardiac electrical signalby first sensing channel 83 used for sensing R-waves. The first sensingvector selected at block 502 for obtaining the cardiac electrical signalused for sensing R-waves may be a relatively short bipole, e.g., betweenelectrodes 28 and 30 or between electrodes 28 and 24 of lead 16 or otherelectrode combinations as described above. The first sensing vector maybe a vertical sensing vector (with respect to an upright or standingposition of the patient) or approximately aligned with the cardiac axisfor maximizing the amplitude of R-waves in the cardiac electrical signalfor reliable R-wave sensing. In other examples, the first sensing vectormay be a vector between one electrode carried along the distal portion25 of lead 16 and the ICD housing 15 (shown in FIG. 1A).

Sensing circuit 86 may produce an R-wave sensed event signal at block506 in response to the first sensing channel 83 detecting an R-wavesensing threshold crossing by the cardiac electrical signal outside of ablanking period. The R-wave sensed event signal may be passed to controlcircuit 80. In response to the R-wave sensed event signal, timingcircuit 90 of control circuit 80 determines an RRI at block 510 endingwith the current R-wave sensed event signal and beginning with the mostrecent preceding R-wave sensed event signal. The timing circuit 90 ofcontrol circuit 80 may pass the RRI timing information to thetachyarrhythmia detection circuit 92 which adjusts tachyarrhythmiainterval counters at block 512.

If the RRI is shorter than a tachycardia detection interval (TDI) butlonger than a fibrillation detection interval (FDI), i.e., if the RRI isin a tachycardia detection interval zone, a VT interval counter isincreased at block 512. If the VT interval counter is configured tocount consecutive VT intervals for detecting VT, the VT interval countermay be reset to zero if the RRI is longer than the TDI. If the RRI isshorter than the FDI, the VF counter is increased. The VF counter may bea probabilistic VF counter that counts VF intervals in an X of Y mannersuch that VF may be detected when a threshold number of VF intervals aredetected which are not required to be consecutive. In some examples, acombined VT/VF interval counter is increased if the RRI is less than theTDI.

After updating the tachyarrhythmia interval counters at block 512,tachyarrhythmia detector 92 compares the VT and VF interval countervalues to a suspected fast heart rate threshold at block 514, which isless than the respective VT NID and VF NID. If a threshold number ofshort RRIs are counted, the onset of a fast heart rate is suspected. Ifa VT or VF detection interval counter has reached fast heart ratethreshold, “yes” branch of block 514, control circuit 80 enables cardiacsignal segment buffering at block 604. In this example, thedetermination of morphology match scores between the SVT template andbuffered cardiac signal segments may be performed on an event-by-eventbasis only after at least one of the VT or VF interval counter valueshas reached fast heart rate threshold. In addition or alternatively toapplying a fast heart rate threshold to the individual VT and VFcounters, a fast heart rate threshold may be applied to a combined VT/VFinterval counter. The fast heart rate threshold may be a value of one ormore. Different fast heart rate thresholds may be applied to the VTinterval counter and the VF interval counter. For example, the fastheart rate threshold may be a count of two on the VT interval counterand a count of three on the VF interval counter. In other examples, thefast heart rate threshold is a higher number, for example five orhigher, but may be less than the number of intervals required to detectVT or VF.

If the fast heart rate threshold is not reached by any of thetachyarrhythmia interval counters at block 514, the control circuit 80returns to block 506 and waits for the next R-wave sensed event signal.Morphology analysis of cardiac signal segments from the second cardiacelectrical signal need not be performed until at least threshold numberof VT or VF intervals is counted as an indication of a suspected fastheart rate and in anticipation of an NID being reached. In this way,control circuit 80 may be able to make a determination of whether theVT/VF morphology criteria are satisfied and acquire data for SVTdiscrimination by the time an NID is reached by either one of the VT orVF interval counters.

If the fast heart rate threshold is reached at block 514, the controlcircuit 80 enables waveform buffering at block 604. In response to eachR-wave sensed event signal produced at block 506 by the first sensingchannel 83, control circuit 80 buffers a cardiac electrical signalreceived by the second sensing channel 85. The sensing circuit 86selects a second sensing vector at block 602 for receiving the cardiacsignal that is buffered for obtaining cardiac signal segments formorphology analysis and SVT discrimination.

A digitized segment of the cardiac electrical signal received by thesecond sensing channel 85 may be buffered over a time segment definedrelative to the sample point time of the R-wave sensing thresholdcrossing and corresponding R-wave sensed event signal received fromsensing circuit 86. The digitized segment may be 100 to 500 ms long, forinstance. In one example, the buffered segment of the second cardiacelectrical signal is at least 48 sample points obtained at a samplingrate of 256 Hz, or approximately 188 ms, of which 24 sample points mayprecede and include the sample point at which the R-wave sensed eventsignal was received and 24 sample points may extend after the samplepoint at which the R-wave sensed event signal was received. In otherexamples, the cardiac electrical signal segment may be buffered at block604 over a longer time interval for use in other cardiac signal analysesperformed to detect noise in the cardiac signal, T-wave oversensing, orother sensing issues that may lead to a false VT or VF detection.

The buffered cardiac signal segment may be notch filtered at block 605.The notch filter applied at block 605 may correspond to the filterdescribed in provisional U.S. Patent Application No. 62/367,166,incorporated herein by reference in its entirety. The notch filteringperformed at block 605 significantly attenuates 50-60 Hz electricalnoise, muscle noise, other EMI, and other noise/artifacts in the storedcardiac signal segment from the second cardiac electrical signal.

In one example, notch filtering performed at block 605 is implemented infirmware as a digital integer filter. The output of the digital notchfilter may be determined by firmware implemented in the second sensingchannel 85 according to the equation:Y(n)=(x(n)+2x(n−2)+x(n−4))/4

where x(n) is the amplitude of the nth sample point of the digitalsignal received by the notch filter 76 (FIG. 4), x(n−2) is the amplitudeof the n−2 sample point, and x(n−4) is the amplitude of the n−4 samplepoint for a sampling rate of 256 Hz. Y(n) is the amplitude of the nthsample point of the notch-filtered, digital second cardiac electricalsignal. At a frequency of 60 Hz, the attenuation of the magnitude ofY(n) is −40 decibels (dB). At a frequency of 50 Hz, the attenuation is−20 dB, and at 23 Hz, which may be typical of an R-wave of the cardiacelectrical signal, the attenuation is limited to −3 dB. Notch filteringat block 605 may therefore provide highly attenuated 50 and 60 Hz noise,muscle noise, other EMI, and other electrical noise/artifacts whilepassing lower frequency cardiac signals in the cardiac electrical signaloutput of second sensing channel 85.

The sample point numbers indicated in the equation above for determininga notch-filtered signal may be modified as needed when a differentsampling rate other than 256 Hz is used, and the resulting frequencyresponse may differ somewhat from the example given above. In otherexamples, other digital filters may be used for attenuation of 50 and 60Hz. For example, for a sampling rate of 256 Hz, a filtered signal Y(n)may be determined as Y(n)=(x(n)+x(n−1)+x(n−2)+x(n−3))/4 which may haverelatively less attenuation at 50 and 60 Hz but acts as a low-pass,notch filter with relatively greater attenuation at higher frequencies(greater than 60 Hz).

Under the control of control circuit 80, a predetermined number ofcardiac signal segments may be stored in memory 82 in a rolling,first-in-first-out buffer. In the illustrative examples describedherein, eight cardiac signal segments are buffered in memory 82. Atblock 606, control circuit 80 may determine morphology match scores foreach of the buffered cardiac signal segments on a beat-by-beat basis aseach signal segment is stored. The morphology match score may bedetermined by comparing wavelet transform coefficients determined from agiven cardiac signal segment to the wavelet transform coefficients of apreviously established SVT template, e.g., as described in conjunctionwith FIG. 6. Other techniques for determining a morphology match scoremay be used.

SVT discrimination features may also be derived from each cardiac signalsegment that is buffered in memory 82. The six SVT discriminationfeatures described in conjunction with FIGS. 7-9 may be determined foreach stored signal segment. As a new cardiac signal segment is stored,the oldest signal segment, along with its morphology match score and SVTdiscrimination features may be deleted.

At block 608, control circuit 80 determines if VT/VF morphology criteriaare met using the buffered morphology match scores determined at block606. Once the buffer for storing eight cardiac signal segments andcorresponding morphology match scores and SVT discrimination features isfilled, control circuit 80 determines if at least X out of the Ymorphology match scores, e.g., at least 6 out of 8, are less than afirst match threshold. If fewer than the threshold number (orpercentage) of cardiac signal segments have a morphology match scorethat is less than the match threshold, the VT/VF morphology criteria arenot met. This result indicates that at least Y-X cardiac signal segmentshave a relatively high correlation to the SVT template and is evidencethat the rhythm is supraventricular. If the VT/VF morphology criteriaare not met (“no” branch of block 608), a morphology rejection rule maybe set at block 610. When requirements for setting this rejection ruleare satisfied, a VT or VF detection may be withheld when a VT or VFinterval counter reaches a respective NID.

If the VT/VF morphology criteria are met at block 608, control circuit80 may determine if SVT discrimination criteria are met at block 612.SVT discrimination may be enabled at block 612 when the VT/VF morphologycriteria are met and at least X of the Y buffered cardiac signalsegments are greater than an SVT discrimination match threshold. Forexample, if at least 6 out of 8 match scores were less than a firstmatch threshold at block 608, but at least 6 out of 8 match scores aregreater than an SVT discrimination match threshold that is less than thefirst match threshold, SVT discrimination criteria are met and SVTdiscrimination is enabled at block 612.

Control circuit 80 determines if SVT detection criteria are satisfied atblock 614 if SVT discrimination is enabled at block 612. The SVTdiscrimination features determined from each notch-filtered cardiacsignal segment are used to determine if SVT detection criteria are met.A first portion of the features determined for each cardiac signalsegment is used to determine if the Y cardiac signal segments aremonomorphic waveforms. A second portion of the features determined foreach cardiac signal segment is used to determine if SVT beat criteriaare met. If the monomorphic waveform criteria are satisfied and the SVTbeat criteria are satisfied, SVT detection criteria are satisfied atblock 614. The SVT rejection rule is set at block 616. Control circuit80 may set a rejection rule by setting a bit value stored in a registeror memory 82 to a high value, e.g., set to 1, to indicate the rejectionrule is satisfied and a VT/VF detection based on NID being met (and/orother detection criteria) should be rejected. If the rejection rule isnot set, e.g., a corresponding register bit value being low or zero, aVT/VF detection is not withheld based on the rejection rule.

If the morphology rejection rule is set at block 610, in response to theVT/VF morphology criteria not being met at block 608, or the SVTrejection rule is set at block 616, control circuit 80 may adjust VT/VFmorphology criteria at block 618. The VT/VF morphology criteria may beadjusted to increase the time that is required to detect VT or VFbecause of the evidence of a supraventricular rhythm associated with themorphology rejection rule being set and/or the SVT rejection rule beingset. The VT/VF morphology criteria may be adjusted to increase thenumber of cardiac signal segments required to have a morphology matchingscore less than the first match threshold before VT or VF can bedetected again. For example, a VT/VF morphology counter that counts downto zero as sensed R-waves are classified as potential VT/VF beats may beadjusted to an increased value, e.g., to a value of five or ten asdescribed in conjunction with FIG. 10. In other examples, a VT/VFmorphology counter may start at zero and count up as sensed R-waves areclassified as potential VT/VF beats. The counter may be reset to zero atblock 618 and/or a threshold count value required for satisfying VT/VFmorphology criteria may be increased, e.g., from an initial threshold ofone to a threshold of five or ten potential VT/VF beats.

If an NID is not reached by one of the VT, VF or combined VT/VF intervalcounters at block 516, control circuit 80 returns to block 506 to sensethe next R-wave, determine the next RRI for updating the intervalcounters, and buffer the next cardiac signal segment if the. Themorphology rejection rule and the SVT rejection rule may be updated on abeat-by-beat basis according to the analysis of the new group ofbuffered cardiac signal segments.

If the NID is reached at block 516 by one of the VT, VF or combinedVT/VF interval counters, control circuit 80 checks at block 518 if arejection rule has been satisfied. If the morphology rejection rule isset, VT or VF detection is withheld at block 524 even though the NID hasbeen reached. If the SVT rejection rule is set, the VT or VF detectionis withheld at block 524, even if the VT/VF morphology criteria havebeen met at block 608 and the NID is reached at block 516. No therapy isdelivered. Control circuit 80 advances to the next sensed R-wave tocontinue updating the VT and VF interval counters and analyzing the nextgroup of buffered cardiac signal segments to update the status of themorphology and SVT rejection rules.

If the NID is reached and the neither the morphology rejection rule northe SVT rejection rule are set, meaning that the VT/VF morphologycriteria are met and the SVT detection criteria are unmet (or SVTdiscrimination is not enabled), VT or VF is detected at block 520according to the interval counter that reached its respective NID.Control circuit 80 controls therapy delivery circuit 84 to deliver atherapy at block 522, which may include ATP, a CV/DF shock, and/orpost-shock pacing pulses in some examples.

FIG. 14 is a flow chart 700 of a method for detecting ventriculartachyarrhythmias according to another example. Identically-numberedblocks in FIG. 14 correspond to like-numbered blocks described inconjunction with FIG. 13. As described above, VT/VF morphology criteriamay be met when X of Y morphology match scores are less than the firstmatch threshold and the VT/VF morphology counter is at a zero value. TheVT/VF morphology counter may be initialized to a zero value such that asingle sensed R-wave classified as a potential VT/VF beat based on agroup of Y morphology match scores satisfies the VT/VF morphologycriteria initially.

As shown FIG. 14, after a fast heart rate threshold is reached by a VTor VF interval counter as determined at block 514, cardiac signalsegments are stored in memory 82 in a rolling buffer. The morphologymatch scores and SVT discrimination features of the buffered cardiacsignal segments are determined beat-by-beat at block 606. If at least Xof Y morphology match scores determined for each buffered cardiac signalsegment are less than the first match threshold at block 708, the VT/VFmorphology counter is decreased by one at block 710 (unless already at azero value). If fewer than X of Y morphology match scores are less thanthe first match threshold, indicating a relatively high correlationbetween the signal segments and the SVT template, the VT/VF morphologycriteria are adjusted by increasing the value of the VT/VF counter atblock 712. In one example, each time less than X of Y morphology matchscores are less than the first match threshold, the VT/VF morphologycounter is set to a value of 10 or another selected value greater thanzero.

After either decreasing the VT/VF counter at block 710 or adjusting theVT/VF counter to a non-zero value at block 712, control circuit 80determines if an NID is reached by any of the VT, VF or combined VT/VFinterval counters being updated at block 512. If an NID is reached,control circuit 80 determines if the VT/VF morphology counter is at avalue of zero at block 714. If not, VT/VF morphology criteria have notbeen met and the process returns to block 506 to wait for the nextR-wave sensed event signal.

If the VT/VF morphology counter is at a value of zero at block 714,control circuit 80 determines if SVT discrimination criteria are met bydetermining if at least X of Y match scores stored in memory 82 for thebuffered cardiac signal segments are greater than the second matchthreshold, which is lower than the first match threshold, at block 715.If not, the Y cardiac signal segments have very low correlation to theSVT template. No SVT discrimination is needed. The VT/VF detectioncriteria are satisfied based on an NID being reached at block 516 andthe VT/VF morphology criteria being satisfied at block 714. VT or VF isdetected at block 520.

If at least X of Y morphology match scores are greater than the second,SVT discrimination threshold at block 715, control circuit 80 determinesif SVT detection criteria are met at block 716. As described previously,e.g., in conjunction with FIGS. 11 and 12, SVT discrimination featuresdetermined from the buffered cardiac signal segments at block 606 areanalyzed to determine if monomorphic waveform criteria are met and ifSVT beat criteria are met. If at least one of the monomorphic waveformcriteria or the SVT beat criteria are not satisfied, the SVT detectioncriteria are not met at block 716 (“no” branch). VT or VF is detected atblock 520, and an appropriate therapy is delivered at block 522. Inother examples, SVT detection criteria are met when one of themonomorphic waveform criteria or the SVT beat criteria are met so thatSVT detection criteria are not met at block 716 only when both of themonomorphic waveform criteria and the SVT beat criteria are not met.

If SVT detection criteria are met at block 716, e.g., both themonomorphic waveform criteria and the SVT beat criteria are satisfied,the VT/VF morphology criteria are adjusted at block 718 by setting theVT/VF morphology counter to a non-zero value, e.g., to a value of five.VT/VF detection is effectively withheld and delayed by requiring theVT/VF morphology counter to count back down to a value of zero before VTor VF can be detected. In order to return to a value of zero in theillustrative examples presented herein, the VT/VF morphology countermust be decreased by one at block 710 on five consecutively sensedR-waves based on X of Y most recent morphology match scores being lessthan the first match threshold on each of the five consecutively sensedR-waves. If SVT detection criteria are met at block 716, VT/VF detectionis withheld at block 524 after adjusting the VT/VF counter, and notherapy is delivered even though both the NID was reached at block 516and the VT/VF morphology criteria were met based on a VT/VF morphologycounter value of zero at block 714.

Thus, techniques for withholding a VT or VF detection based on cardiacsignal segment features satisfying SVT detection criteria, even whenboth RRI-based and waveform morphology-based VT/VF detection criteriaare satisfied, are presented herein. The cardiac signal segment featureanalysis for SVT discrimination avoids false VT or VF detections in thesituation of an altered cardiac signal morphology due to positionalchanges of the patient's body or posture or other factors that mayinfluence the cardiac signal waveforms such as R-waves (or QRScomplexes). The techniques disclosed herein may be implemented inconjunction with additional VT/VF rejection rules that cause a VT or VFdetection to be withheld based on additional analysis of the bufferedcardiac signal segments. Various cardiac signal analysis techniques andVT/VF rejection rules that may be implemented in conjunction with theSVT discrimination techniques disclosed herein are generally disclosedin provisionally filed U.S. Patent Application No. 62/367,166 , U.S.Patent Application No. 62/367,170 , U.S. Patent Application No.62/367,221 and U.S. patent application Ser. No. 15/140,802 (Zhang, etal.), all of which are incorporated herein by reference in theirentirety. Additional analysis may be performed for detectingelectromagnetic interference or other noise in the cardiac electricalsignal, T-wave oversensing or verifying sensed R-waves. These additionalanalyses may be used for other rejection rules for withholding a VT orVF detection when an NID is reached. As such, control circuit 80 maycheck the status of multiple rejection rules at block 518 of FIG. 13, asgenerally disclosed in the above-incorporated patent applications.

Thus, an ICD system and method for discriminating SVT from ventriculartachyarrhythmias and withholding a ventricular tachyarrhythmia detectionand therapy in response to detecting SVT have been presented in theforegoing description with reference to specific embodiments. In otherexamples, various methods described herein may include steps performedin a different order or different combination than the illustrativeexamples shown and described herein. It is appreciated that variousmodifications to the referenced embodiments may be made withoutdeparting from the scope of the disclosure and the following claims.

The invention claimed is:
 1. An implantable cardioverter defibrillator(ICD) comprising: a therapy delivery circuit configured to generate anelectrical stimulation therapy for delivery to a patient's heart; asensing circuit configured to receive at least a first cardiacelectrical signal via a sensing electrode vector; and a control circuitcoupled to the sensing circuit and the therapy delivery circuit andconfigured to: determine whether first criteria for detecting aventricular tachyarrhythmia are met by the first cardiac electricalsignal; determine a plurality of features from each one of a pluralityof cardiac signal segments of the first cardiac electrical signal; inresponse to the first criteria being met, determine whether a firstportion of the plurality of features determined from each one of theplurality of cardiac signal segments satisfy monomorphic waveformcriteria; determine whether a second portion of the plurality offeatures determined from each one of the plurality of cardiac signalsegments satisfy supraventricular beat criteria by comparing eachfeature of the second portion of the features determined from each ofthe plurality of cardiac signal segments to an analogous feature of asupraventricular R-wave template and determining that thesupraventricular beat criteria are satisfied in response to a thresholdnumber of the cardiac signal segments having the second portion of thefeatures matching the analogous features of the supraventricular R-wavetemplate; determine whether second criteria for detecting theventricular tachyarrhythmia are met; withhold detecting of theventricular tachyarrhythmia in response to both the monomorphic waveformcriteria and the supraventricular beat criteria being satisfied; anddetect the ventricular tachyarrhythmia and controlling the therapydelivery circuit to deliver the electrical stimulation therapy inresponse to the first criteria and the second criteria being met and atleast one of the monomorphic waveform criteria not being satisfied orthe supraventricular beat criteria not being satisfied.
 2. The system ofclaim 1, wherein the control circuit is configured to determine whetherthe first criteria are met by: determining a morphology match scorebetween each one of a plurality of cardiac signal segments of the firstcardiac signal and a morphology template; determining that the firstcriteria are met in response to a first threshold number of theplurality of cardiac signal segments having a morphology match scoreless than a first match threshold.
 3. The system of claim 2, wherein thecontrol circuit is further configured to: in response to the firstcriteria being met, determine whether supraventricular tachyarrhythmiadiscrimination criteria are met; withhold determining the plurality offeatures in response to the supraventricular tachyarrhythmiadiscrimination criteria not being met; and detect the ventriculartachyarrhythmia in response to the first and second criteria fordetecting the ventricular tachyarrhythmia being met and thesupraventricular tachyarrhythmia discrimination criteria not being met.4. The system of claim 3, wherein the control circuit is configured: todetermine that the supraventricular tachyarrhythmia discriminationcriteria are met by determining that at least a second threshold numberof the plurality of cardiac signal segments have a morphology matchscore greater than a second match threshold, the second match thresholdless than the first match threshold.
 5. The system of claim 2, whereinthe control circuit is further configured to: set a threshold countvalue in response to less than the threshold number of the plurality ofcardiac signal segments having a morphology match score less than thefirst match threshold; adjust a tachyarrhythmia count value in responseto the threshold number of a next plurality of cardiac signal segmentshaving morphology match scores less than the first match threshold; anddetermine that the first criteria are met when the tachyarrhythmia countvalue reaches the threshold count value.
 6. The system of claim 1,wherein the control circuit is further configured to: determine sensedevent intervals between consecutive R-waves sensed by the sensingcircuit; compare the sensed event intervals to a tachyarrhythmiadetection interval; increase a count of tachyarrhythmia detectionintervals in response to each one of the determined sensed eventintervals that is less than the tachyarrhythmia detection interval; andin response to a value of the count of tachyarrhythmia detectionintervals being equal to or greater than a fast heart rate threshold,determine whether the first criteria for detecting the ventriculartachyarrhythmia are met.
 7. The system of claim 1, wherein the controlcircuit is configured to: determine the features for each one of theplurality of cardiac signal segments by determining at least a polaritypattern, a peak time interval; and a normalized width for each one ofthe plurality of cardiac signal segments; and determine whether thesupraventricular beat criteria are satisfied by: determining, for eachof the plurality of cardiac signal segments, whether the determinedpolarity pattern matches a polarity pattern of a supraventricular R-wavetemplate; determining, for each of the plurality of cardiac signalsegments, whether the determined peak time interval matches a peak timeinterval of the supraventricular R-wave template within a peak timeinterval match threshold range; determining, for each one of theplurality of cardiac signal segments, whether the determined normalizedwidth matches a normalized width of the supraventricular R-wave within anormalized width match threshold range; and determining that thesupraventricular beat criteria are satisfied in response to a thresholdnumber of the cardiac signal segments having the determined features ofpolarity pattern, maximum peak amplitude and normalized width matchingthe analogous features of the supraventricular R-wave template.
 8. Thesystem of claim 1, wherein the control circuit is configured todetermine whether the monomorphic waveform criteria are satisfied by:determining a variability of each of the features of the first portionof the features determined from each of the plurality of cardiac signalsegments; comparing the variability of each of the features of the firstportion of the features to a respective variability threshold;determining that the monomorphic waveform criteria are satisfied inresponse to the variability of each of the features of the first portionof the features being less than the respective variability threshold. 9.The system of claim 1, wherein the control circuit is configured to:determine a polarity of a maximum peak of a supraventricular R-wavetemplate; determine the features for each one of the plurality ofcardiac signal segments by determining at least an amplitude and timingof a maximum peak of the cardiac signal segment that has a polaritymatching the polarity of the maximum peak of the supraventricular R-wavetemplate; determine an amplitude variability and a timing variability ofthe maximum peaks of the plurality of cardiac signal segments; anddetermine that the monomorphic waveform criteria are satisfied inresponse to the amplitude variability being less than an amplitudevariability threshold and the timing variability being less than atiming variability threshold.
 10. The system of claim 1, wherein thecontrol circuit is further configured to: determine event intervalsbetween consecutive events sensed by the sensing circuit; compare theevent intervals to a tachyarrhythmia detection interval; increase acount of tachyarrhythmia detection intervals in response to each one ofthe determined event intervals that is less than the tachyarrhythmiadetection interval; determine that the second criteria for detecting theventricular tachyarrhythmia are met in response to a value of the countof tachyarrhythmia detection intervals being equal to or greater than adetection threshold value.
 11. The system of claim 1, wherein: thesensing circuit comprises a first sensing channel for receiving thefirst cardiac signal and a second sensing channel for receiving a secondcardiac signal and configured to sense R-waves from the second cardiacsignal and produce an R-wave sensed event signal in response to eachsensed R-wave; the control circuit is configured to: determine a sensedevent interval between each consecutive pair of R-wave sensed eventsignals produced by the sensing circuit; compare the sensed eventintervals to a tachyarrhythmia detection interval; increase a count oftachyarrhythmia detection intervals in response to each one of thedetermined sensed event intervals that is less than the tachyarrhythmiadetection interval; buffer the plurality of cardiac signal segments fromthe first cardiac signal in response to a value of the count oftachyarrhythmia detection intervals being equal to or greater than afast heart rate threshold, each one of the plurality of cardiac signalsegments corresponding to an R-wave sensed event signal produced by thesensing circuit; and determine that the second criteria for detectingthe ventricular tachyarrhythmia are met in response to the count of thetachyarrhythmia detection intervals being equal to or greater than adetection threshold value.
 12. The system of claim 1, wherein thecontrol circuit is further configured to adjust the first criteria inresponse to both of the monomorphic waveform criteria and thesupraventricular beat criteria being satisfied.
 13. An implantablecardioverter defibrillator, comprising: a therapy delivery circuitconfigured to generate an electrical stimulation therapy for delivery toa patient's heart; a sensing circuit configured to receive at least afirst cardiac electrical signal via a sensing electrode vector; acontrol circuit coupled to the sensing circuit and the therapy deliverycircuit and configured to: determine whether first criteria fordetecting a ventricular tachyarrhythmia are met by the first cardiacelectrical signal; determine a plurality of features from each one of aplurality of cardiac signal segments of the first cardiac electricalsignal; in response to the first criteria being met, determine whether afirst portion of the plurality of features determined from each one ofthe plurality of cardiac signal segments satisfy monomorphic waveformcriteria; determine whether a second portion of the plurality offeatures determined from each one of the plurality of cardiac signalsegments satisfy supraventricular beat criteria by comparing eachfeature of the second portion of the features determined from each ofthe plurality of cardiac signal segments to an analogous feature of asupraventricular R-wave template and determining that thesupraventricular beat criteria are satisfied in response to a thresholdnumber of the cardiac signal segments having the second portion of thefeatures matching the analogous features of the supraventricular R-wavetemplate; determine whether second criteria for detecting theventricular tachyarrhythmia are met withhold detecting of theventricular tachyarrhythmia in response to both the monomorphic waveformcriteria and the supraventricular beat criteria being satisfied; anddetect the ventricular tachyarrhythmia and controlling the therapydelivery circuit to deliver the electrical stimulation therapy inresponse to the first criteria and the second criteria being met and atleast one of the monomorphic waveform criteria not being satisfied orthe supraventricular beat criteria not being satisfied; and a housingenclosing the therapy delivery circuit, the sensing circuit and thecontrol circuit, the housing having a connector block for receiving anextra-cardiovascular lead carrying at least one electrode of the sensingelectrode vector.
 14. A method comprising: receiving by a sensingcircuit at least a first cardiac electrical signal via a sensingelectrode vector; determining by a control circuit whether firstcriteria for detecting a ventricular tachyarrhythmia are met by thefirst cardiac electrical signal; determining a plurality of features foreach one of a plurality of cardiac signal segments of the first cardiacelectrical signal; in response to the first criteria being met,determining whether a first portion of the plurality of featuresdetermined from each one of the plurality of cardiac signal segmentssatisfy monomorphic waveform criteria; determining whether a secondportion of the plurality of features determined from each one of theplurality of cardiac signal segments satisfy supraventricular beatcriteria by comparing each feature of the second portion of the featuresdetermined from each of the plurality of cardiac signal segments to ananalogous feature of a supraventricular R-wave template and determiningthat the supraventricular beat criteria are satisfied in response to athreshold number of the cardiac signal segments having the secondportion of the features matching the analogous features of thesupraventricular R-wave template; determining whether second criteriafor detecting the ventricular tachyarrhythmia are met; determiningwhether both the first portion of the plurality of features satisfy themonomorphic waveform criteria and the second portion of the plurality offeatures satisfy the supraventricular beat criteria; withholdingdetecting of the ventricular tachyarrhythmia in response both the firstportion of the plurality of features satisfying the monomorphic waveformcriteria and the second portion of the plurality of features satisfyingthe supraventricular beat criteria; and detecting the ventriculartachyarrhythmia and controlling the therapy delivery circuit to deliverthe electrical stimulation therapy in response to the first criteria andthe second criteria being met and at least one of the first portion ofthe plurality of features not satisfying the monomorphic waveformcriteria and/or the second portion of the plurality of features notsatisfying the supraventricular beat criteria.
 15. The method of claim14, wherein determining whether the first criteria are met comprises:determining a morphology match score between each one of a plurality ofcardiac signal segments of the first cardiac signal and a morphologytemplate; determining that the first criteria are met in response to afirst threshold number of the plurality of cardiac signal segmentshaving a morphology match score less than a first match threshold. 16.The method of claim 15, further comprising: in response to the firstcriteria being met, determining whether supraventricular tachyarrhythmiadiscrimination criteria are met; withholding determining the pluralityof features in response to the supraventricular tachyarrhythmiadiscrimination criteria not being met; and detecting the ventriculartachyarrhythmia in response to the first and second criteria fordetecting the ventricular tachyarrhythmia being met and thesupraventricular tachyarrhythmia discrimination criteria not being met.17. The method of claim 16, further comprising: determining that thesupraventricular tachyarrhythmia discrimination criteria are met bydetermining that at least a second threshold number of the plurality ofcardiac signal segments have a morphology match score greater than asecond match threshold, the second match threshold less than the firstmatch threshold.
 18. The method of claim 15, further comprising: settinga threshold count value in response to less than the threshold number ofthe plurality of cardiac signal segments having a morphology match scoreless than the first match threshold; adjusting a tachyarrhythmia countvalue in response to the threshold number of a next plurality of cardiacsignal segments having morphology match scores less than the first matchthreshold; and determining that the first criteria are met when thetachyarrhythmia count value reaches the threshold count value.
 19. Themethod of claim 14, further comprising: determining sensed eventintervals between consecutive R-waves sensed by the sensing circuit;comparing the sensed event intervals to a tachyarrhythmia detectioninterval; increasing a count of tachyarrhythmia detection intervals inresponse to each one of the determined sensed event intervals that isless than the tachyarrhythmia detection interval; and in response to avalue of the count of tachyarrhythmia detection intervals being equal toor greater than a fast heart rate threshold, determining whether thefirst criteria for detecting the ventricular tachyarrhythmia are met.20. The method of claim 14, wherein: determining the features for eachone of the plurality of cardiac signal segments comprises determining atleast a polarity pattern, a peak time interval; and a normalized widthfor each one of the plurality of cardiac signal segments; anddetermining whether the supraventricular beat criteria are satisfiedcomprises: determining, for each of the plurality of cardiac signalsegments, whether the determined polarity pattern matches a polaritypattern of a supraventricular R-wave template; determining, for each ofthe plurality of cardiac signal segments, whether the determined peaktime interval matches a peak time interval of the supraventricularR-wave template within a peak time interval match threshold range;determining, for each one of the plurality of cardiac signal segments,whether the determined normalized width matches a normalized width ofthe supraventricular R-wave within a normalized width match thresholdrange; and determining whether a threshold number of the cardiac signalsegments have the determined features of polarity pattern, maximum peakamplitude and normalized width matching the analogous features of thesupraventricular R-wave template.
 21. The method of claim 14, whereindetermining whether the monomorphic waveform criteria are satisfiedcomprises: determining a variability of each of the features of thefirst portion of the plurality of features determined from each of theplurality of cardiac signal segments; comparing the variability of eachof the features of the first portion to a respective variabilitythreshold; and determining that the monomorphic waveform criteria aresatisfied in response to the variability of each of the features of thefirst portion of the plurality of features being less than therespective variability threshold.
 22. The method of claim 14, furthercomprising: determining a polarity of a maximum peak of asupraventricular R-wave template; determining the features for each oneof the plurality of cardiac signal segments by determining at least anamplitude and timing of a maximum peak of the cardiac signal segmentthat has a polarity matching the polarity of the maximum peak of thesupraventricular R-wave template; determining an amplitude variabilityand a timing variability of the maximum peaks of the plurality ofcardiac signal segments; and determining that the monomorphic waveformcriteria are satisfied in response to the amplitude variability beingless than an amplitude variability threshold and the timing variabilitybeing less than a timing variability threshold.
 23. The method of claim14, further comprising: determining event intervals between consecutiveevents sensed by the sensing circuit; comparing the event intervals to atachyarrhythmia detection interval; increasing a count oftachyarrhythmia detection intervals in response to each one of thedetermined event intervals that is less than the tachyarrhythmiadetection interval; determining that the second criteria for detectingthe ventricular tachyarrhythmia are met in response to a value of thecount of tachyarrhythmia detection intervals being equal to or greaterthan a detection threshold value.
 24. The method of claim 14, furthercomprising: receiving the first cardiac signal by a first sensingchannel of the sensing circuit; receiving a second cardiac signal by asecond sensing channel of the sensing circuit; sensing R-waves from thesecond cardiac signal; producing an R-wave sensed event signal inresponse to each sensed R-wave, determining a sensed event intervalbetween each consecutive pair of the R-wave sensed event signalsproduced by the sensing circuit; comparing the sensed event intervals toa tachyarrhythmia detection interval; increasing a count oftachyarrhythmia detection intervals in response to each one of thedetermined sensed event intervals that is less than the tachyarrhythmiadetection interval; buffering the plurality of cardiac signal segmentsfrom the first cardiac signal in response to a value of the count oftachyarrhythmia detection intervals being equal to or greater than asensed event confirmation threshold, each one of the plurality ofcardiac signal segments corresponding to an R-wave sensed event signalproduced by the sensing circuit; and determining that the secondcriteria for detecting the ventricular tachyarrhythmia are met inresponse to the count of the tachyarrhythmia detection intervals beingequal to or greater than a detection threshold value.
 25. The method ofclaim 14, further comprising adjusting the first criteria in response toboth of the monomorphic waveform criteria and the supraventricular beatcriteria being satisfied.
 26. The method of claim 14, further comprisingreceiving the first cardiac electrical signal via the sensing electrodevector comprising at least one electrode carried by anextra-cardiovascular lead.
 27. A non-transitory, computer-readablestorage medium comprising a set of instructions which, when executed bya control circuit of an implantable cardioverter defibrillator (ICD),cause the ICD to: receive by a sensing circuit a cardiac electricalsignal via a sensing electrode vector; determine whether first criteriafor detecting a ventricular tachyarrhythmia are met by the cardiacelectrical signal; determine a plurality of features for each one of aplurality of cardiac signal segments of the cardiac electrical signal;in response to the first criteria being met, determine whether a firstportion of the plurality of features determined from each one of theplurality of cardiac signal segments satisfy monomorphic waveformcriteria; determine whether a second portion of the plurality offeatures determined from each one of the plurality of cardiac signalsegments satisfy supraventricular beat criteria by comparing eachfeature of the second portion of the features determined from each ofthe plurality of cardiac signal segments to an analogous feature of asupraventricular R-wave template and determining that thesupraventricular beat criteria are satisfied in response to a thresholdnumber of the cardiac signal segments having the second portion of thefeatures matching the analogous features of the supraventricular R-wavetemplate; determine whether second criteria for detecting theventricular tachyarrhythmia are met; withhold detecting of theventricular tachyarrhythmia in response to both the monomorphic waveformcriteria being satisfied and the supraventricular beat criteria beingsatisfied; and detect the ventricular tachyarrhythmia and deliver anelectrical stimulation therapy by a therapy delivery circuit in responseto the first criteria and the second criteria being met and at least oneof the monomorphic waveform criteria not being satisfied or thesupraventricular beat criteria not being satisfied.